usb: phy: mxs: Fix NULL pointer dereference on i.MX23/28

usb: phy: mxs: Fix NULL pointer dereference on i.MX23/28

Commit e93650994a95 ("usb: phy: mxs: add usb charger type detection")
causes the following kernel hang on i.MX28:

[    2.207973] usbcore: registered new interface driver usb-storage
[    2.235659] Unable to handle kernel NULL pointer dereference at virtual address 00000188
[    2.244195] pgd = (ptrval)
[    2.246994] [00000188] *pgd=00000000
[    2.250676] Internal error: Oops: 5 [#1] ARM
[    2.254979] Modules linked in:
[    2.258089] CPU: 0 PID: 1 Comm: swapper Not tainted 4.15.0-rc8-next-20180117-00002-g75d5f21 #7
[    2.266724] Hardware name: Freescale MXS (Device Tree)
[    2.271921] PC is at regmap_read+0x0/0x5c
[    2.275977] LR is at mxs_phy_charger_detect+0x34/0x1dc

mxs_phy_charger_detect() makes accesses to the anatop registers via regmap,
however i.MX23/28 do not have such registers, which causes a NULL pointer
dereference.

Fix the issue by doing a NULL check on the 'regmap' pointer.

Fixes: e93650994a95 ("usb: phy: mxs: add usb charger type detection")
Cc: <stable@vger.kernel.org> # v4.15
Reviewed-by: Li Jun <jun.li@nxp.com>
Acked-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>