i2c: zynq: Support for 0-length register address

Fixed bug with alen == 0 in 'i2c_write', 'i2c_read'
Further minor corrections:
- Write 'address' register before 'data' register.
- Write 'transfer_size' register before 'address' register.

Signed-off-by: Michael Burr <michael.burr@logicpd.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>