V4L/DVB: pvrusb2: Avoid using stack allocated buffers when performing USB I/O

Drivers shouldn't assume that the stack is DMA-safe.

[mchehab@redhat.com: fix patch description]
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>