i2c: qup: Fix wrong value of index variable

index gets incremented during check to determine if the
messages can be transferred with dma. But not reset after
that, resulting in wrong start value in subsequent loop,
causing failure. Fix it.

Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org