gianfar: Fix a locking bug in gianfar's sysfs code

During sparse cleanup, found a locking bug.  Some of the sysfs functions were
acquiring a lock, and then returning in the event of an error.  We rearrange
the code so that the lock is released in error conditions, too.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>