cmsis_dap: fix build on macOS

cmsis_dap: fix build on macOS

Compile fails with error:
	src/jtag/drivers/cmsis_dap.c:683:28: error: format specifies type
	    'unsigned char' but the argument has type 'int' [-Werror,-Wformat]
	                         " received 0x%" PRIx8, CMD_DAP_TFER, resp[0]);
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

Fix the format specifier.

Change-Id: I0a5a1a35452d634019989d14d849501fb8a7e93a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6255
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>