NET: am79c961: fix race in link status code

NET: am79c961: fix race in link status code

The link status code operates from a timer, and writes the index
register without first taking a lock.  A well-placed interrupt
between writing the index register and reading the data register
could change the index register on us, which will return wrong data.
Add the necessary lock.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>