net: phy: smsc: prepare for making edpd wait period configurable

Add a member edpd_max_wait_ms to the private data structure in preparation
of making the wait period configurable by supporting the edpd phy tunable.

v2:
- rename constant to EDPD_MAX_WAIT_DFLT_MS

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>