smsc911x: enforce read-after-write timing restriction on eeprom access

The LAN911x datasheet specifies a minimum delay of 45ns between a write
of E2P_DATA and any read.  This patch adds a single dummy read of
BYTE_TEST to enforce this timing constraint.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>