i2c-msm-v2: correct throughput calculation in i2c driver

Calculation of throughput should not consider overhead bytes
as a result of tags. Only actual data should be used
to calculate the I2C throughput values.

Change-Id: I605a5284a3deef1ade4a70f064ac7af3b7b3edf6
Signed-off-by: Ankit Gupta <ankgupta@codeaurora.org>