media: dvb_usb_pctv452e: module refcount changes were unbalanced

dvb_frontend will call dvb_detach for:
- stb0899_detach in dvb_frontend_release and
- stb0899_release in __dvb_frontend_free

But we only do dvb_attach(stb0899_attach).
Increment the module refcount instead.

Signed-off-by: Wolfgang Rohdewald <wolfgang@rohdewald.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>