usb: phy: phy-generic: Fix USB PHY gpio reset

Since commit e9f2cefb ("usb: phy: generic: migrate to gpio_desc") a
kernel hang is observed on imx51-babbage board:

[    1.392824] ci_hdrc ci_hdrc.1: doesn't support gadget
[    1.397975] ci_hdrc ci_hdrc.1: EHCI Host Controller
[    1.403205] ci_hdrc ci_hdrc.1: new USB bus registered, assigned bus number 1
[    1.422335] ci_hdrc ci_hdrc.1: USB 2.0 started, EHCI 1.00
[    1.432962] hub 1-0:1.0: USB hub found
[    1.437119] hub 1-0:1.0: 1 port detected

This hang happens because the reset GPIO stays at logic level 0.

The USB PHY reset gpio is defined in the dts file as:

reset-gpios = <&gpio2 5 GPIO_ACTIVE_LOW>;

, which means it is active low, so what the gpio reset pin needs to do in this
case is the following:

- Go to logic level 0 to reset the USB PHY
- Stay at 0 for a bit
- Go back to logic level 1

When switching to gpiod API we need to following according to
Documentation/gpio/consumer.txt:

"The first thing a driver must do with a GPIO is setting its direction. If no
direction-setting flags have been given to gpiod_get*(), this is done by
invoking one of the gpiod_direction_*() functions:

	int gpiod_direction_input(struct gpio_desc *desc)
	int gpiod_direction_output(struct gpio_desc *desc, int value)"

Since no direction-setting flags have been given to devm_gpiod_get_optional()
in our case, we need to use gpiod_direction_output to comply with the gpiod API.

With this change the USB PHY reset performs a proper reset, the kernel boots
fine and USB host is functional.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>