mmc: sdhci-msm: Avoid overiding of vendor_specific_func register

For HS400 tuning in HS200 mode MCLK should be configured to 384 MHz in GCC.
The clock on the bus should be 192MHz which is achieved by selecting MCLK/2
in VENDOR_SPECIFIC_FUNC by setting SDC4_MCLK_SEL field values as 3.

But the above register configurations are getting overridden with older
values leading to tuning failure for HS400 mode.

Avoid overiding of vendor_specific_func register.

Change-Id: I8ff094f6fa5019a0b42ed9716e52b9fd9c085934
Signed-off-by: Sarthak Garg <sartgarg@codeaurora.org>