drivers: xilinx_spi: Use udevice in start_tranfer

Modify start_transfer and related functions to take a udevice parameter
as input instead of spi_slave. This is needed so that start_transfer can
be used directly via the xfer callback. Also fix a compiler warning.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Tested-by: Love Kumar <love.kumar@amd.com>
Link: https://lore.kernel.org/r/20231116164336.140171-2-mchitale@ventanamicro.com


Signed-off-by: Michal Simek <michal.simek@amd.com>