phy: phy-imx8mq-usb: Remove .exit operation

phy: phy-imx8mq-usb: Remove .exit operation

Currently, when running "ums 0 mmc 2" and breaking it via
CTRL + C, the following message is seen:

u-boot=> ums 0 mmc 1
UMS: LUN 0, dev mmc 1, hwpart 0, sector 0x0, count 0x1dacc00
CTRL+C - Operation aborted
clk usb_phy_root_clk already disabled

The USB PHY clock is disabled twice: first it gets disabled
inside imx8mq_usb_phy_power_off(), then it is disabled again
inside imx8mq_usb_phy_exit().

Let the USB PHY clock be disabled only once inside
imx8mq_usb_phy_power_off() by removing the .exit operation.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>