powerpc/usb: fix bug of CPU hang when missing USB PHY clock

when missing USB PHY clock, u-boot will hang during USB
initialization when issuing "usb start". We should check
USBGP[PHY_CLK_VALID] bit to avoid CPU hanging in this case.

Due to controller issue of PHY_CLK_VALID in ULPI mode, we set
USB_EN before checking PHY_CLK_VALID, otherwise PHY_CLK_VALID
doesn't work.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>