smsc95xx: check return code from control messages

This patch adds additional checks of the values returned by
smsc95xx_(read|write)_reg, and wraps their common patterns
in macros.
Signed-off-by: NSteve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>