greybus: gpio: fix truncated debounce times

Fix set_debounce, which silently truncated the time argument to 255us
even though we support 16-bit values.

Also remove the unnecessary explicit cast when verifying the argument.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>