net/smsc911x: Correctly configure 16-bit register access from DT

The SMSC911X_USE_16BIT needs to be set when using 16-bit register
access.  However, currently no flag is set if the device tree
doesn't specify 32-bit access, resulting in a BUG() and a non-
working driver when 16-bit register access is configured for
smsc911x in the DT.

This patch should set the SMSC911X_USE_16BIT flag in a manner
consistent with the documented DT bindings.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>