[media] em28xx: Add a separate config dir for secondary bus

Prepare to register a separate bus for the second bus.
For now, just add a new field. A latter patch will add the
bits to make it work.
This patch was generated by this script:
perl -e 'while (<>) { if (s/EM2874_I2C_SECONDARY_BUS_SELECT.*\n//) {
	printf "\t\t.def_i2c_bus  = 1,\n"; $found = 1; print $_ } else { if ($found) { s/^\s+// }; $found = 0; print $_; } }' \
drivers/media/usb/em28xx/em28xx-cards.c >a && mv a drivers/media/usb/em28xx/em28xx-cards.c

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>