AX88796: Fix locking in ethtool support

Fix a pair of nasty locking problems in the ax88796 driver
spotted by a sparse check:

warning: context imbalance in 'ax_get_settings' - wrong count at exit
warning: context imbalance in 'ax_set_settings' - wrong count at exit

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>