USB: opticon: fix non-atomic allocation in write path

commit e681286d upstream.

Write may be called from interrupt context so make sure to use
GFP_ATOMIC for all allocations in write.

Fixes: 0d930e51 ("USB: opticon: Add Opticon OPN2001 write support")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>