USB: serial: mos7720: fix non-atomic allocation in write path

There is an allocation with GFP_KERNEL flag in mos7720_write(),
while it may be called from interrupt context.

Follow-up for commit 19125283 ("USB: kobil_sct: fix non-atomic
allocation in write path")

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>