media: lgdt3306a: Fix module count mismatch on usb unplug

When used as an i2c device there is a module usage count mismatch on
removal, preventing the driver from being used thereafter. dvb_attach
increments the usage count so it is properly balanced on removal.

On disconnect of Hauppauge SoloHD/DualHD before:

lsmod | grep lgdt3306a
lgdt3306a              28672  -1
i2c_mux                16384  1 lgdt3306a

On disconnect of Hauppauge SoloHD/DualHD after:

lsmod | grep lgdt3306a
lgdt3306a              28672  0
i2c_mux                16384  1 lgdt3306a

Signed-off-by: Brad Love <brad@nextdimension.cc>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>