usb: chipidea: imx: simplify the usbmisc callers

Move struct imx_usbmisc_data NULL pointer judgement from caller to
each API, it can simplify the caller.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>