mmc: sdhci-msm: Shift the read minor revision value

This is required to read the proper minor revision value
from the nvmem.
(Documentation/devicetree/bindings/nvmem/nvmem.txt)
Nvmem reads 30 bits with bit offset 0. To read bits 28-29
the consumers have to right shift it by 28.

Change-Id: Ie66f47dfadfbb0360cd10272b836898aa7858263
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>