jtag: drivers: buspirate: chunk SWD switch sequence transfer.

jtag: drivers: buspirate: chunk SWD switch sequence transfer.

Commit c2e18bfaeafd changed the size of the JTAG-to-SWD sequence
from 15 bytes to 17 bytes. This broke SWD switch sequence transfer
for buspirate, since buspirate packets can only hold a payload of up
to 16 bytes and we tried to fit the whole sequence in a single packet.

Splitting up the sequence transfer in appropriately sized packets
makes buspirate SWD work again (successfully tested with buspirate
firmwares v6.1 and v7.0).

Change-Id: Ib5b412b9e77287d705d2762e31c16d30318b50e3
Signed-off-by: Tilman Sauerbeck <tilman@code-monkey.de>
Reviewed-on: http://openocd.zylin.com/5200
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>