net: qualcomm: rmnet: always expose a few functions

A recent change tidied up some conditional code, avoiding the use of
some #ifdefs.  Unfortunately, if CONFIG_IPV6 was not enabled, it
meant that two functions were referenced but never defined.

The easiest fix is to just define stubs for these functions if
CONFIG_IPV6 is not defined.  This will soon be simplified further
by some other development in the works...

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 75db5b07

 ("net: qualcomm: rmnet: eliminate some ifdefs")
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>