mmc: sdhci-msm: do not disable test-bus

There is a hardware ring-buffer that logs the commands
sent and doorbells rung. This is helpful for debugging
in case of errors. This requires that the testbus be
enabled. Hence don't disable testbus.

Change-Id: I9c2fa984740aa9a0f8135d2196be6b3639ec22d1
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>