media: ddbridge: only register frontends in fe2 if fe is not NULL

Smatch reported:

  drivers/media/pci/ddbridge/ddbridge-core.c:1602 dvb_input_attach() error: we previously assumed 'dvb->fe' could be null (see line 1595)

dvb->fe2 will ever only be populated when dvb->fe is set. So only handle
registration of dvb->fe2 when dvb->fe got set beforehand by moving the
registration into the "if (dvb->fe)" conditional.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Tested-by: Jasmin Jessich <jasmin@anw.at>
Tested-by: Dietmar Spingler <d_spingler@freenet.de>
Tested-by: Manfred Knick <Manfred.Knick@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>