V4L/DVB (7319): pvrusb2: Close potential race condition during initialization

There is a callback that is issued to into pvr2_context from pvr2_hdw
after initialization is done.  There was a probability that this
callback could get missed.  Fixed.

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