staging: rtl8187se: Use ARRAY_SIZE instead of in-code constants

Use ARRAY_SIZE macro instead of hardcoding array size into loop
condition and remove unused last item of rtl8225z2_rxgain[]

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>