USB: serial: usb_wwan: Add missing uaccess.h / fix build failure

This patch fixes a build failure[1] by adding the missing uaccess.h needed
for copy_from_user and copy_to_user

References:
	http://kisskb.ellerman.id.au/kisskb/buildresult/3607218/

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>