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

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")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>