generic: platform/mikrotik: release mtd device after use

generic: platform/mikrotik: release mtd device after use

The code uses get_mtd_device_nm() which must be followed by a call to
put_mtd_device() once the handle is no longer used.

This fixes spurious shutdown console messages such as:
[ 2256.334562] Removing MTD device #7 (soft_config) with use count 1

Reported-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
(cherry picked from commit 4e385a27d6c4aab62ef7fc9856f09ebee9632985)