usb: gadget: m66592-udc needs linux/err.h.

In certain configurations linux/err.h is not included through alternate
means, resulting in:

drivers/usb/gadget/m66592-udc.c:1646: error: implicit declaration of function 'IS_ERR'
drivers/usb/gadget/m66592-udc.c:1649: error: implicit declaration of function 'PTR_ERR'
distcc[15083] ERROR: compile drivers/usb/gadget/m66592-udc.c on localhost failed
make[3]: *** [drivers/usb/gadget/m66592-udc.o] Error 1
make[2]: *** [drivers/usb/gadget] Error 2
make[1]: *** [drivers] Error 2
make: *** [sub-make] Error 2

Caught with an ARM config in -next.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>