[media] au0828: fix possible race condition in usage of dev->ctrlmsg

The register read function is referencing the dev->ctrlmsg structure outside
of the dev->mutex lock, which can cause corruption of the value if multiple
callers are invoking au0828_readreg() simultaneously.

Use a stack variable to hold the result, and copy the buffer returned by
usb_control_msg() to that variable.

In reality, the whole recv_control_msg() function can probably be collapsed
into au0288_readreg() since it is the only caller.

Also get rid of cmd_msg_dump() since the only case in which the function is
ever called only is ever passed a single byte for the response (and it is
already logged).

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>