net: allow shifted access in smsc911x V2

This is a revised patch that permits a shifted access to the
LAN9221 registers.  More specifically:

 It adds a shift parameter in the platform_data.
 It introduces an ops in smsc911x_data.
 A choice of access function to use at run-time.
 Four new shifted access function.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>