staging: wfx: gpiod_get_value() can return an error

Smatch complains:

    hif_rx.c:98 hif_wakeup_indication() warn: 'gpiod_get_value(wdev->pdata.gpio_wakeup)' returns positive and negative
    bh.c:24 device_wakeup() warn: 'gpiod_get_value_cansleep(wdev->pdata.gpio_wakeup)' returns positive and negative

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20201009171307.864608-7-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>