i2c: qup: minor code reorganization for use_dma

1. Assigns use_dma in qup_dev structure itself which will
   help in subsequent patches to determine the mode in IRQ handler.
2. Does minor code reorganization for loops to reduce the
   unnecessary comparison and assignment.

Signed-off-by: Abhishek Sahu <absahu@codeaurora.org>
Reviewed-by: Austin Christ <austinwc@codeaurora.org>
Reviewed-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>