smc91x: introduce platform data flags V2

This patch introduces struct smc91x_platdata and modifies the driver so
bus width is checked during run time using SMC_nBIT() instead of
SMC_CAN_USE_nBIT.

V2 keeps static configuration lean using SMC_DYNAMIC_BUS_CONFIG.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>