mmc: omap_hsmmc: Fix the DT pbias workaround for MMC controllers 2 to 5

Otherwise SDIO cards won't necessarily work when booted with
device tree as we will never power down the SDIO cards. This
means the SDIO card reset does not happen which at least some
WLAN controllers expect to happen with ifconfig wlan0 down.

The PBIAS voltage is only available for the first controller
instance, so let's limit the PBIAS workaround to the first
controller only.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Tested-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>