tplink-safeloader: fix C7v5 factory flashing from vendor fw > v1.1.x

tplink-safeloader: fix C7v5 factory flashing from vendor fw > v1.1.x

Currently it's not possible to flash factory images on devices shipped
with vendor firmware versions 1.1.0 Build 20201120 rel. 50406 (published
2020-12-22):

 (curFw_ver, newFw_ver) == (1.1, 1.0) [NM_Error](nm_checkSoftVer) 00848: Firmwave not supports, check failed.
 [NM_Error](nm_checkUpdateContent) 01084: software version dismatched
 [NM_Error](nm_buildUpgradeStruct) 01188: checkUpdateContent failed.

They've even following note in release notes:

 Note: You will be unable to downgrade to the previous firmware version
       after updating this firmware.

This version check in vendor firmware is implemented in
/usr/bin/nvrammanager binary likely as following C code[1]:

 sscanf(buf, "%d.%d.%*s",&upd_fw_major, &upd_fw_minor);
 ...
 if (((int)upd_fw_major < (int)cur_fw_major) ||
     ((ret = 1, cur_fw_major == upd_fw_major && (upd_fw_minor < (int)cur_fw_minor)))) {
       ret = 0;
       printf("[NM_Error](%s) %05d: Firmwave not supports, check failed.\r\n\r\n","nm_checkSoftVer" ,0x350);
 }
 ...
 return ret;

So in order to fix this and make it future proof it should be enough to
ship our factory firmware images with major version 7 (lucky number).

Tested on latest firmware version 1.1.2 Build 20210125 rel.37999:

 Firmwave supports, check OK.
  (curFw_ver, newFw_ver) == (1.1, 7.0) check firmware ok!

Flashing back to vendor firmware
c7v5_us-up-ver1-1-2-P1[20210125-rel37999]_2021-01-25_10.33.55.bin works
as well:

 U-Boot 1.1.4-gbec22107-dirty (Nov 18 2020 - 18:19:12)
 ...
 Firmware downloaded... filesize = 0xeeae77 fileaddr = 0x80060000.
 Firmware Recovery file length : 15642231
 Firmware process id 2.
 handle_fw_cloud 146
 Image verify OK!
 Firmware file Verify ok!
 product-info:product_name:Archer C7
 product_ver:5.0.0
 special_id:55530000
 [Error]sysmgr_cfg_checkSupportList(): 1023 @ specialId 45550000 NOT Match.
 Firmware supports, check OK.
 Firmware Recovery check ok!

1. https://gist.github.com/ynezz/2e0583647d863386a66c3d231541b6d1

Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit e6d66375cbbb54e0e82a67030e385a5486273766)