powermate_alloc_buffers() is never called in atomic context. It calls
usb_alloc_coherent() with GFP_ATOMIC, which is not necessary. GFP_ATOMIC
can be replaced with GFP_KERNEL.

This is found by a static analysis tool named DCNS written by myself.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>