V4L/DVB (5172): Pvrusb2: Control protocol cleanup

Several special-case FX2 commands were being issued through
pvr2_write_u16() and pvr2_write_8(), but there's really nothing
special case about them.  These date from a very early time in the
driver development.  This patch removes these functions and replaces
their use with calls to pvr2_send_request.

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