media: em28xx-dvb: simplify DVB module probing logic

The module probing logic there is a way more complex than
it should be, and requires some special magic to avoid
stack overflows when KASAN is enabled.

Solve it by creating ancillary functions to setup the
platform data and request module.

Now, the probing functions are cleaner and easier to understand.

As a side effect, the size of the module was reduced by
about 9.7% on x86_64:

Before this patch:
   text	   data	    bss	    dec	    hex	filename
  51090	  14192	     96	  65378	   ff62	drivers/media/usb/em28xx/em28xx-dvb.o

After this patch:
   text	   data	    bss	    dec	    hex	filename
  44743	  14192	     96	  59031	   e697	drivers/media/usb/em28xx/em28xx-dvb.o

Tested with a PCTV 461e device.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>