i2c-msm-v2: correct and improve clock control calculation

On some targets t-low is below i2c spec for 400KHz. Also,
clock control value is calulated per transfer, when calculting
it once is sufficient. This patch warns when device tree configures
an unsuppurted bus frequency and removed related unused debugfs
entries.

Change-Id: I5f721426c7deeac31ee20512816759f1c6407f80
Signed-off-by: Sana Venkat Raju <c_vsana@codeaurora.org>
Signed-off-by: Ankit Gupta <ankgupta@codeaurora.org>