OMAP2+: TWL: make conversion routines static

The uv_to_vsel, vsel_to_uv functions don't need to be exposed to the
world as they are used as function pointers. make them static.

Fixes sparse warnings:
arch/arm/mach-omap2/omap_twl.c:63:15: warning: symbol 'twl4030_vsel_to_uv' was not declared. Should it be static?
arch/arm/mach-omap2/omap_twl.c:68:4: warning: symbol 'twl4030_uv_to_vsel' was not declared. Should it be static?
arch/arm/mach-omap2/omap_twl.c:73:15: warning: symbol 'twl6030_vsel_to_uv' was not declared. Should it be static?
arch/arm/mach-omap2/omap_twl.c:105:4: warning: symbol 'twl6030_uv_to_vsel' was not declared. Should it be static?

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>