V4L/DVB (7712): pvrusb2: Close connect/disconnect race

If a disconnect happens before initialization is completed, the
pvrusb2 driver can accidentally touch dangling pointers.  The whole
initialization function must be protected by the big_lock, and once
inside that lock, the initialization function should abort if it is
discovered that a disconnect has already taken place.

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