staging: wfx: fix test on return value of gpiod_get_value()

The commit 8522d62e ("staging: wfx: gpiod_get_value() can return an
error") has changed the way the driver test the value returned by
gpiod_get_value(). The new code was wrong.

Fixes: 8522d62e

 ("staging: wfx: gpiod_get_value() can return an error")
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20201019160604.1609180-2-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>