smsc95xx: remove unnecessary variables

Removes unnecessary variables as smsc95xx_write_reg takes its
value by parameter.  Early versions passed this parameter by
reference.

Also replace hardcoded interrupt status value with a #define

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