tree/: Drop default_spi_probe_opcode for NULL case

A NULL func pointer is necessary and sufficient for the
condition `NULL func pointer => true' as to not need this
boilerplate as it implies default behaviour of a supported
opcode within the `check_block_eraser()` match supported loop.

Ran;
```
$  find . -name '*.[c,h]' -exec sed -i '/.probe_opcode	= default_spi_probe_opcode,/d' '{}' \;
```

Change-Id: Id502c5d2596ad1db52faf05723083620e4c52c12
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/70264
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>

digilent_spi.c

@@ -336,7 +336,6 @@ static const struct spi_master spi_master_digilent_spi = {
 	.read		= default_spi_read,
 	.write_256	= default_spi_write_256,
 	.shutdown	= digilent_spi_shutdown,
-	.probe_opcode	= default_spi_probe_opcode,
 };
 
 static bool default_reset(struct libusb_device_handle *handle)