drivers/: Make 'internal_delay' the default unless defined

Drop the explicit need to specify the default 'internal_delay'
callback function pointer in the programmer_entry struct.
This is a reasonable default for every other driver in the
tree with only the two exceptions of ch341a_spi.c and serprog.c.

Thus this simplifies driver development.

Change-Id: I17460bc2c0aebcbb48c8dfa052b260991525cc49
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/67391
Reviewed-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Nikolai Artemiev <nartemiev@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

realtek_mst_i2c_spi.c

@@ -546,5 +546,4 @@ const struct programmer_entry programmer_realtek_mst_i2c_spi = {
 	.init			= realtek_mst_i2c_spi_init,
 	.map_flash_region	= fallback_map,
 	.unmap_flash_region	= fallback_unmap,
-	.delay			= internal_delay,
 };