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

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

commit 5a5a1d614287a647b36dff3f40c2b0ceabbc83ec upstream.

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

Follow-up for commit 191252837626 ("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>
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>