usb: chipidea: usbmisc_imx: Staticize usbmisc_imx_drv_init/exit

Fix the following sparse warnings:

drivers/usb/chipidea/usbmisc_imx.c:246:5: warning: symbol 'usbmisc_imx_drv_init' was not declared. Should it be static?
drivers/usb/chipidea/usbmisc_imx.c:252:6: warning: symbol 'usbmisc_imx_drv_exit' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>