net: macb: Use variables with defaults for tx/rx ring sizes instead of hardcoded values

The macb driver hardcoded the tx/rx ring sizes. This made it
impossible to change the sizes at run time.

Add tx_ring_size, and rx_ring_size variables to macb object, which
are initilized with default vales during macb_init. Change all
references to RX_RING_SIZE and TX_RING_SIZE to their respective
replacements.

Signed-off-by: Zach Brown <zach.brown@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>