bnxt_en: Add bnxt_get_total_vnics() to calculate number of VNICs

Refactor the code by adding a new function to calculate the number of
required VNICs.  This is used in multiple places when reserving or
checking resources.

Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Venkat Duvvuru <venkatkumar.duvvuru@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>