staging: mt7621-gpio: avoid devm_kzalloc() hidden inside declarations and refactor function a bit

Driver probe function includes an allocation using devm_kzalloc
which is "hidden" a bit inside the declarations. Extract this
to a better place to increase readability. Also because we are
allocating zeroed memory 'memset' statement is not needed at all.
Condition for checking for a valid gpio id is wrong and it should
be greater or equal instead of only greater so update to be the
good one.

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