spi: zynq: Introduce a timeout to the transfer completion

If the interrupt never kicks in, e.g. due to a peripheral malfunctions, the
driver will get stuck forever in xspips_start_transfer() for the completion to
complete. This patch adds a timeout to the completion so the driver can fail
gracefully and return an error if the interrupt never kicks in.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>