usb: pass flags parameter to gpiod_get functions

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.

Currently this parameter is made optional with the help of a cpp trick.
To allow dropping this hack convert callers to explictly pass a value
for flags.

Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>