staging: mt7621-gpio: change gc_map to don't use pointers

There is no special gain in using pointers for 'gc_map' inside
'mtk_data' structure. We know the number of banks which is fixed
to MTK_BANK_CNT and we can just statically allocate them without
using kernel allocators.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>