usb: dwc3: pci: make better use of gpiod API

Since 39b2bbe3 (gpio: add flags argument to gpiod_get*() functions)
which appeared in v3.17-rc1, the gpiod_get* functions take an additional
parameter that allows to specify direction and initial value for output.

Use this additional parameter and the _optional variant to simplify the
driver and improve error handling. Also expand the comment to explain
why it's not sensible to switch to devm_gpiod_get and why the gpiod_put
is also necessary.

Furthermore this is one caller less that stops us making the flags
argument to gpiod_get*() mandatory.

Tested-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>