rsi: fix non-atomic allocation in completion handler

USB completion handlers are called in atomic context and must
specifically not allocate memory using GFP_KERNEL.

Fixes: a1854fae

 ("rsi: improve RX packet handling in USB interface")
Cc: stable <stable@vger.kernel.org> # 4.17
Cc: Prameela Rani Garnepudi <prameela.j04cs@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>