OMAP: VP: Explicitly mask VPVOLTAGE field

Reading the VPVOLTAGE field of PRM_VP_*_VOLTAGE registers currently
relies on a u32 -> u8 conversion to mask off the FORCEUPDATEWAIT field
in the upper bits.  Make this explicit using the mask symbol
already defined, added as a new field in struct omap_vp_common.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>