scsi: ufs: serialize h8 and device command contexts

There's a race between clock scaling and issuing dev
commands. Both these contexts can run simultaneously.
Add a synchronization for this to avoid hibernating
during scaling while issuing dev commands.

Change-Id: Ic53f59d65d480a1857798ddc492028d3b756ef84
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>