idpf: remove unreachable code from setting mailbox

Remove code that isn't reached. There is no need to check for
adapter->req_vec_chunks, because if it isn't set idpf_set_mb_vec_id()
won't be called.

Only one path when idpf_set_mb_vec_id() is called:
idpf_intr_req()
 -> idpf_send_alloc_vectors_msg() -> adapter->req_vec_chunk is allocated
 here, otherwise an error is returned and idpf_intr_req() exits with an
 error.

The idpf_set_mb_vec_id() becomes one-liner and it is called only once.
Remove it and set mailbox vector index directly.

Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Reviewed-by: Pavan Kumar Linga <pavan.kumar.linga@intel.com>
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Samuel Salin <Samuel.salin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>