bnxt_en: Refactor bnxt_free_rx_skbs().

bnxt_free_rx_skbs() frees all the allocated buffers and SKBs for
every RX ring.  Refactor this function by calling a new function
bnxt_free_one_rx_ring_skbs() to free these buffers on one specified
RX ring at a time.  This is preparation work for resetting one RX
ring during run-time.

Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>