rtos: fixed handling of qThreadExtraInfo packets

The commit "gdbserver: use common hexify/unhexify routines" [3d62c3d]
mis-replaced a call to "str_to_hex" with a call to "unhexify". "hexify"
should have been used instead.

Change-Id: I5f5904b1b422f819a6308e2c0740ea43d22c7d0b
Signed-off-by: Christian Gudrian <christian.gudrian@gmx.de>
Reviewed-on: http://openocd.zylin.com/1308
Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>