[NET] smc91x: prepare for SMC_IO_SHIFT to be a platform configurable variable

Now one can use the following code

  #define SMC_IO_SHIFT	lp->io_shift

to make SMC_IO_SHIFT a variable. This, however, will slightly increase
the CPU overhead and have negative impact on the network performance.
The tradeoff is, this can be specified in the smc91x platform data so
that multiple boards support can be built in a single zImage.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Acked-by: Nicolas Pitre <nico@cam.org>
Acked-by: Jeff Garzik <jgarzik@pobox.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>