usb: chipidea: usbmisc_imx: fix a possible NULL dereference

of_match_device could return NULL, and so cause a NULL pointer
dereference later. Renaming tmp_dev to of_id (like all others do) in the
process.

Reported-by: coverity (CID 1324135)
Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>