treewide: fix hex2dec conversion for MAC address checksum offset

treewide: fix hex2dec conversion for MAC address checksum offset

If chksum_offset is converted by $(($...)) at the beginning, the
check [ -n "$chksum_offset" ] will always return true, as the
conversion yields "0" for an empty argument, and [ -n "0" ] is
true.

With this patch, the variable is not converted before the check,
but only when it's used in dd.

No conversion is done for use in hexdump, as this can deal with
hex value offsets.

Fixes: b133e466b08e ("treewide: convert WiFi caldata size and offset to hexadecimal")
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>