arm_adi_v5: Fix packed transfers crossing TAR auto-increment block

arm_adi_v5: Fix packed transfers crossing TAR auto-increment block

The word count returned from max_tar_block_size() was compared with the
count of half-word/bytes in the u16 and u8 packed access functions,
causing an infinite loop if the access actually crossed the boundary.

Change max_tar_block_size() to return a byte count, and scale at the call
site.

Change-Id: I2fe9b5941eb485f3d8219cfdd29fb71e02006de4
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1649
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>