V4L/DVB (7701): pvrusb2: Centralize handling of simple FX2 commands

Numerous places in the driver need to issue simple commands to the FX2
microcontroller (e.g. only 1 or 2 bytes, no reply needed).  Previously
each place that did this, had to take lock, set up a central buffer,
and call the function to perform the handshake.  This change puts
these steps into a single spot.  This also has the effect of removing
the need to mess with the control lock from numerous places in the
code.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>