drivers/: Make 'fallback_{un}map' the default unless defined

Drop the explicit need to specify the default 'fallback_{un}map'
callback function pointer from the 'programmer_entry' struct.
This is a reasonable default for every other driver in the tree
with only a select few exceptions [atavia, serprog, dummyflasher
and internal].

Thus this simplifies driver development and paves way
to remove the 'programmer' global handle.

Change-Id: I5ea7bd68f7ae2cd4af9902ef07255ab6ce0bfdb3
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/67404
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

ft2232_spi.c

@@ -711,6 +711,4 @@ const struct programmer_entry programmer_ft2232_spi = {
 	.type			= USB,
 	.devs.dev		= devs_ft2232spi,
 	.init			= ft2232_spi_init,
-	.map_flash_region	= fallback_map,
-	.unmap_flash_region	= fallback_unmap,
 };