net: smc91x: Fix device tree based configuration so it's usable

Commit 89ce376c

 (drivers/net: Use of_match_ptr() macro in smc91x.c)
added minimal device tree support to smc91x, but it's not working on
many platforms because of the lack of some key configuration bits.

Fix the issue by parsing the necessary configuration like the
smc911x driver is doing. As most smc91x users seem to use 16-bit
access, let's default to that if no reg-io-width is specified.

Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: netdev@vger.kernel.org
Cc: devicetree@vger.kernel.org
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>