staging: mt7621-gpio: use devm_kasprintf to set gpio banks labels

Instead of using a custom function to return desired name for gpio
use the default assigned one and concat it '-bankN' suffix using
devm_kasprintf kernel function.

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