net: emaclite: update reset_lock member documentation

Instead of generic description, mention what reset_lock actually
protects i.e. lock to serialize xmit and tx_timeout execution.

Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>