spi: zynqmp: gqspi: add checks on clock API return codes

This patch does following:

- Adds checks on clock API return values inside

- Factors the clk_get_rate() out of the loop rather than repeatedly
  calling the clk API

Signed-off-by: Ranjit Waghmode <ranjit.waghmode@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>