staging: mt7621-gpio: rename MTK_MAX_BANK into MTK_BANK_CNT

There are 3 banks of gpios numbered '0' and '1' and '2'. So
the maximum bank number is "2". "3" is the count of banks.
In order to make the code looks and be correct on checking
max allowed gpio's id it makes sense to change the name of
this definition. Also there is another definitions which
start with the same prefix MKK_BANK_ of the new name so
having those with the same prefix makes all preprocessor
structure to be the same. This improves readability.

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