V4L/DVB (9346): Optimization: Enable gate in a symmetric/disciplined way,

rather than implementing different ways leading to confusion.

This allows multiple gate_enable/disable's in the tuner_read/write
functions, thereby lesser number of I/O operations throughout,
eventually leading to better results. As a side effect demods that
detect the STOP bit for auto closing of the gate can be avoided, thereby
a very minimal gain in disabling the auto detect feature as well.
Improves readability on the device control.

Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>