bnxt_en: include bnxt_vfr.c code under CONFIG_BNXT_SRIOV switch

And define empty functions in bnxt_vfr.h when CONFIG_BNXT_SRIOV is not
defined.

This fixes build error when CONFIG_BNXT_SRIOV is switched off:
>> drivers/net/ethernet/broadcom/bnxt/bnxt_vfr.c:165:16: error: 'struct
>> bnxt' has no member named 'sriov_lock'

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 4ab0c6a8 ("bnxt_en: add support to enable VF-representors")
Signed-off-by: Sathya Perla <sathya.perla@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>