gianfar: Add flow control support

eTSEC has Rx and Tx flow control capabilities that may be enabled
through MACCFG1[Rx_Flow, Tx_Flow] bits.  These bits must not be set
however when eTSEC is operated in Half-Duplex mode.  Unfortunately,
the driver currently sets these bits unconditionally.
This patch adds the proper handling of the PAUSE frame capability
register bits by implementing the ethtool -A interface.  When pause
autoneg is enabled, the controller uses the phy's capability to
negotiate PAUSE frame settings with the link partner and reconfigures
its Rx_Flow and Tx_Flow settings to match the capabilities of the
link partner.  If pause autoneg is off, the PAUSE frame generation
may be forced manually (ethtool -A).  Flow control is disabled by
default now.
This implementation is inspired by the tg3 driver.

Signed-off-by: Lutz Jaenicke <ljaenicke@innominate.com>
Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>