mmc: omap: Fix a section warning regression

Commit b6e0703b

 (mmc: omap: make it behave well as a module) made some
__devinit changes but missed one function causing a section warning:

WARNING: vmlinux.o(.devinit.text+0x8604): Section mismatch in reference
from the function mmc_omap_probe)

The function __devinit mmc_omap_probe() references a function
__init mmc_omap_new_slot().

Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>