bnxt_en: Allocate rx_cpu_rmap only if Accelerated RFS is enabled.

Also, no need to check for bp->rx_nr_rings as it is always >= 1.  If the
allocation fails, it is not a fatal error and we can still proceed.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>