mmc: omap_hsmmc: Fix pbias_disable for omap4

pbias_disable is set to protect the mmc pbias i/o cells in DT boot
by preventing voltage switch. Currently pbias_disable is enabled only
for omap3 and not for omap4 due to reg_offset difference of 0x100.
Enable pbias_disable for omap4+ too by using res->start
which does not include the reg_offset.

Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>