net: smc91x: take into account register shift

This aligns smc91x with its cousin, namely smc911x.c.
This also allows the driver to run also in a device-tree based lubbock
board build, on which it was tested.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>