adapter: replace 'interface' with 'adapter driver'

adapter: replace 'interface' with 'adapter driver'

Comments and output strings still reference the term 'interface',
while 'adapter driver' should be used.

While there, drop the useless test if CMD_ARGV[0] is an empty
string, as this is not possible when CMD_ARGC > 0.

Change-Id: I7b46b5dd3cec53d8b5b7559d941ee9ae3bd1d89b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8808
Tested-by: jenkins
Reviewed-by: zapb <dev@zapb.de>