USB: bcma: switch to GPIO descriptor for power control

So far we were using simple (legacy) GPIO functions & some poor logic to
control power. It got many drawbacks: we were ignoring OF flags
(GPIO_ACTIVE_LOW), we were not setting direction to output and we were
assuming gpio_request success all the time.
Fix it by switching to gpiod functions and adding appropriate checks.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>