SPI: zynq: Updated suspend and resume logic

Use a driver state variable to indicate suspend or ready state.
Initialize this to ready in probe function.
Changes in suspend function - disable interrupts,
de-assert chip select, signal completion, disable controller and
change driver state to suspend.
Changes in resume function - do not perform hardware initialization
again. Just change driver state to ready.
Changes in prepare_hardware function - Check for driver state -
If ready enable the controller, else return error.

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