[media] s2255drv: dynamic memory allocation efficiency fix

Driver was allocating a kernel buffer each time it was sending a command.
It is better to allocate this buffer once at startup.

Signed-off-by: Dean Anderson <linux-dev@sensoray.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>