gpio/aspeed: Fix incorrect number of banks

gpio/aspeed: Fix incorrect number of banks

The current calculation for the number of GPIO banks is only correct if
the number of GPIOs is a multiple of 32 (if there were 31 GPIOs we would
currently say there are 0 banks, which is incorrect).

Fixes: 8268a7cd889c6 ('gpio: Add Aspeed driver')
Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20190906062623.13354-1-rashmica.g@gmail.com
Reviewed-by: Joel Stanley <joel@jms.d.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>