bnxt_en: Refactor bnxt_init_one_rx_ring().

bnxt_init_one_rx_ring() includes logic to initialize the BDs for one RX
ring and to allocate the buffers.  Separate the allocation logic into a
new bnxt_alloc_one_rx_ring() function.  The allocation function will be
used later to allocate new buffers for one specified RX ring when we
reset that RX ring.

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>