smsc95xx: set MII_BUSY bit to read/write PHY regs

The device datasheet specifies the BUSY bit must be set when reading
or writing phy registers.  This patch ensures we do that.

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