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 <[e-mail]>
Fixes: efdbd3a5d6e ("usb: phy: mxs: do not set PWD.RXPWD1PT1 for low speed connection")
CC: <[e-mail]> # 4.0
Acked-by: Peter Chen <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/phy/phy-mxs-usb.c

···
 {
 	unsigned int vbus_value;
 
+	if (!mxs_phy->regmap_anatop)
+		return false;
+
 	if (mxs_phy->port_id == 0)
 		regmap_read(mxs_phy->regmap_anatop,
 			ANADIG_USB1_VBUS_DET_STAT,