qup_i2c: Calculate combined write length for multiple write transactions

When a transaction consists of multiple write requests, add up the total
length of the write requests.  If there is a read following the writes,
set the read mode appropriately.

Change-Id: I0c15e14fa4732e4e25f5bb885688662a066b8782
Signed-off-by: Kenneth Heitke <kheitke@codeaurora.org>