usb: phy: mxs: suspend to RAM causes NULL pointer dereference

Triggering suspend to RAM via sysfs on a i.MX28 causes a NULL pointer
dereference. This patch avoids the oops in mxs_phy_get_vbus_status()
by aborting since there is no syscon available.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: efdbd3a5d6e ("usb: phy: mxs: do not set PWD.RXPWD1PT1 for low speed connection")
CC: <stable@vger.kernel.org> # 4.0
Acked-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>