mmc: sdhci: Fix sdhci_do_enable_v4_mode

V4_MODE is Bit-15 of SDHCI_HOST_CONTROL2 register.
Need to perform word access to this register.

Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Fixes: b3f80b43 ("mmc: sdhci: Add sd host v4 mode")
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>