HID: usbhid: Make hid_is_usb() non-inline

By making hid_is_usb() a non-inline function the lowlevel usbhid driver
does not have to be exported anymore.

Also mark the argument as const as it is not modified.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: David Rheinsberg <david.rheinsberg@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>