jtag/drivers/cmsis_dap: introduce packet_usable_size
USB bulk backend needs to avoid zero sized USB packets
sent after each full sized packed for performance reasons.
HID backend uses fixed size HID reports so the full size
of the report can be utilized.
Introduce packet_usable_size to reflect it.
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Change-Id: I34094c9edac5730624480711cbd6aa65883c47c7
Reviewed-on: https://review.openocd.org/c/openocd/+/7363
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>