gp8psk: fix gp8psk_usb_in_op() logic

Commit bc29131ecb10 ("[media] gp8psk: don't do DMA on stack") fixed the
usage of DMA on stack, but the memcpy was wrong for gp8psk_usb_in_op().
Fix it.

From Derek's email:
    "Fix confirmed using 2 different Skywalker models with
     HD mpeg4, SD mpeg2."

Suggested-by: Johannes Stezenbach <js@linuxtv.org>
Fixes: bc29131ecb10 ("[media] gp8psk: don't do DMA on stack")
Tested-by: Derek <user.vdr@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>