usb: phy: phy-mxs-usb: Check the return value from stmp_reset_block()

stmp_reset_block() may fail, so let's check its return value and propagate it
in the case of error.

Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>