bnxt_en: Refactor bnxt_vf_reps_create().

Add a new function bnxt_alloc_vf_rep() to allocate a VF representor.
This function will be needed in subsequent patches to recreate the
VF reps after error recovery.

Signed-off-by: Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>