jtag: align adapter speed code to new structure

jtag: align adapter speed code to new structure

Rename the jtag_XXX functions as adapter_XXX.
Rename internal variables.
Adapt log messages and comments text.

Change-Id: I67f209c67074899cd58bad495055def03f0d1bcf
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6644
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>