mmc: sdhci: Use right API to read/write host control2 register

The SDHCI host control2 register offset is 0x3E which is not
word aligned and is only 16 bits wide. But we use readl/writel
to read/write to it. Use readw/writew to avoid any alignment
issues.

Change-Id: Ibaa87ce46f2a2dd97f9e1c7979adb38e426acd5b
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>