I2C: zynq: Use temp variables for registers

Use temporary variables to store register values. This way nesting
read/write accessors is avoided, which makes the code more readable and
achieves nicer indentation.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>