V4L/DVB (7939): pvrusb2: Remove sysfs interface hackery

Back in the early days of the pvrusb2 driver, the kernel class
mechanism in use for the sysfs interface had no means to pass
per-attribute information to the show / store functions.  This forced
me to implement a horrible ugly thunking mechanism (i.e. infer the
missing data through the use of dedicated cookie cutter bounce
functions).  However now we're using a better mechanism which also
passes enough additional information to the show / store functions
that we no longer need the hack.  So eliminate all the crap.  Yay!
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>