net: fec: fix the error to get the previous BD entry

Bug: error to get the previous BD entry. When the current BD
is the first BD, the previous BD entry must be the last BD,
not "bdp - 1" in current logic.

V4:
  * Optimize fec_enet_get_nextdesc() for code clean.
    Replace "ex_new_bd - ring_size" with "ex_base".
    Replace "new_bd - ring_size" with "base".

V3:
  * Restore the API name because David suggest to use fec_enet_
    prefix for all function in fec driver.
    So, change next_bd() -> fec_enet_get_nextdesc()
        change pre_bd()  -> fec_enet_get_prevdesc()
  * Reduce the two APIs parameters for easy to call.

V2:
  * Add tx_ring_size and rx_ring_size to struct fec_enet_private.
  * Replace api fec_enet_get_nextdesc() with next_bd().
    Replace api fec_enet_get_prevdesc() with pre_bd().

  * Move all ring size check logic to next_bd() and pre_bd(), which
    simplifies the code redundancy.

V1:
  * Add BD ring size check to get the previous BD entry in correctly.

Reviewed-by: Li Frank <B20596@freescale.com>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Acked-by: Frank Li <frank.li@freescale.net>
Signed-off-by: David S. Miller <davem@davemloft.net>