bnxt_en: Fix compile error regression with CONFIG_BNXT_SRIOV not set.

Add a new function bnxt_get_registered_vfs() to handle the work
of getting the number of registered VFs under #ifdef CONFIG_BNXT_SRIOV.
The main code will call this function and will always work correctly
whether CONFIG_BNXT_SRIOV is set or not.

Fixes: 230d1f0d ("bnxt_en: Handle firmware reset.")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>