omap_hsmmc.c: Fix build warning on non-omap3

It was incorrect to always include "asm/arch-omap3/mux.h" constantly.
This introduced warnings on non-omap3 where certain values will conflict
between the various families.  Conditionally guard the inclusion in
order to correct the problem.

Fixes: 6aca17c9 ("drivers: mmc: omap_hsmmc: Fix IO Buffer on OMAP36xx")
Signed-off-by: Tom Rini <trini@konsulko.com>