smsc911x: add dynamic bus configuration

Convert the driver to select 16-bit or 32-bit bus access at runtime,
at a small performance cost.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>