jtag: flush jtag queue after jtag_add_tlr()

jtag: flush jtag queue after jtag_add_tlr()

If the TLR sequence is sent as result of the command "adapter
assert trst" while polling is off, the TLR sequence is not sent out
until a following jtag operation.

Flush the jtag queue before return.

Change-Id: I20efd7137cb7b1d1c4f73c1362cbe4e57aeaae49
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5405
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>