[PATCH] ibm_emac: fix graceful stop timeout handling

This patch fixes graceful stop timeout handling in PPC4xx EMAC driver.

Currently, when we stop TX/RX channels we just do some number of loops
without relying on actual spent time. This has finally bitten me on
one of our systems (heavy network traffic during start up, RX channel
is stopped several times to configure multicast list).

Graceful channel stop can take up to 1 frame time, so I've added
device specific timeout counter which depends on current link speed
and calls to udelay() to really wait required amount of time before
giving up.

Signed-off-by: Eugene Surovegin <ebs@ebshome.net>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>