fpga: altera-ps-spi: Fix getting of optional confd gpio

Currently the driver does not handle EPROBE_DEFER for the confd gpio.
Use devm_gpiod_get_optional() instead of devm_gpiod_get() and return
error codes from altera_ps_probe().

Fixes: 5692fae0 ("fpga manager: Add altera-ps-spi driver for Altera FPGAs")
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Moritz Fischer <mdf@kernel.org>