net: smc911x: only enable for mpr2 on sh.

The smc911x.h is a bit of a mess, not supporting any sort of generic
configuration. For the moment only ARCH_PXA and SH_MAGIC_PANEL_R2 have
suitable definitions, so we reflect this in the Kconfig also.

While there are other SH boards that will likely turn this on in the
2.6.25 time frame, it's not worth trying to stub around at the moment.

Fixes up the allmodconfig build, as noted by akpm.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>