bcm4908: fix calculation of new cferam index

bcm4908: fix calculation of new cferam index

The arithmetic expansion fails when idx becomes a two digit number.
Fix this by relying on expr command.

root@OpenWrt:/# echo $(((028 + 0) % 1000))
/bin/ash: arithmetic syntax error
root@OpenWrt:/# echo $(($(expr 028 + 0) % 1000))
28

Fixes: a6a0b252baa6 ("bcm4908: add sysupgrade support")
Signed-off-by: Sungbo Eo <mans0n@gorani.run>
(cherry picked from commit f4323538501d58298f4df73a034a51375a477cfc)