HID: usbkbd: Avoid GFP_ATOMIC when GFP_KERNEL is possible

usb_kbd_alloc_mem is called in usb_kbd_probe, which is
not in atomic context. So constraints can be relaxed here.

Use GFP_KERNEL instead of GFP_ATOMIC.

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>