Remove unnecessary register access in mxl111sf_ep6_streaming_ctrl()

This code breaks driver operation in kernel 3.3 and later, although
it works properly in 3.2  Disable register access to 0x12 for now.
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>