V4L/DVB (4751): Fix DBV_FE_CUSTOMISE for card drivers compiled into kernel

When a front-end is disabled, card drivers that use it are compiled with
a stub version of the front-end's attach function.  This way they have no
references to the front-end's code and don't need it to be loaded.
If a card driver is compiled into the kernel, and a front-end is a
module, then that front-end is effectively disabled wrt the card driver. 
In this case, the card driver should get the stub version.  This was not
happening.
The stub vs real attach function selection is changed so that when the
front-end is a module the real attach function is only used if the card
driver is a module as well.  This means a module front-end will be
supported by card drivers that are modules and not supported by card
drivers compiled into the kernel.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>