mmc: host: sdhci-msm: Update the DDR_CONFIG register name

The variables which holds the register offset for HC_REG_DDR_CONFIG
register, its name got updated from DDR_CONFIG_2 DDR_CONFIG.
So would need to the updated in all other places as well where all
this register is being accessed.

And there was another register variable with same name DDR_CONFIG_2
which is unused and is masking the compilation errors of not updating
DDR_CONFIG register name across the file. Remove this unused register-
altogether.

Change-Id: I1f232091ea0998e486041dc3af7bab9633accbdf
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>