V4L/DVB: budget: Oops: "BUG: unable to handle kernel NULL pointer dereference"

Never call dvb_frontend_detach if we failed to attach a frontend. This fixes
the following oops, which will be triggered by a missing stv090x module:

[    8.172997] DVB: registering new adapter (TT-Budget S2-1600 PCI)
[    8.209018] adapter has MAC addr = 00:d0:5c:cc:a7:29
[    8.328665] Intel ICH 0000:00:1f.5: PCI INT B -> GSI 17 (level, low) -> IRQ 17
[    8.328753] Intel ICH 0000:00:1f.5: setting latency timer to 64
[    8.562047] DVB: Unable to find symbol stv090x_attach()
[    8.562117] BUG: unable to handle kernel NULL pointer dereference at 000000ac
[    8.562239] IP: [<e08b04a3>] dvb_frontend_detach+0x4/0x67 [dvb_core]

Ref http://bugs.debian.org/575207

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>