f432353850
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: a6a0b252ba ("bcm4908: add sysupgrade support")
Signed-off-by: Sungbo Eo <mans0n@gorani.run>