SPI: zynq: Use auto start and keep track of transmit count

Use auto start instead of manual start.
Use a variable "trans_cnt" to keep track of number of bytes
transmitted and rely on this instead of TXFULL and RXNEMPTY
status bits. This is a solution for the clock domain crossing
issue.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>