target: check return value of register get/set callbacks

target: check return value of register get/set callbacks

- In "reg" TCL command handler, the return value of register get()
  and set() callbacks must be checked, in the same manner as it is
  done in e.g. gdb_set_register_packet() or gdb_get_register_packet().

- Minor cleanup of variable definitions in the "reg" command
  handler.

Change-Id: I8c57e7c087fe31d1abffa3c4d1f79a01af4c9c97
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: http://openocd.zylin.com/6293
Tested-by: jenkins
Reviewed-by: Marc Schink <dev@zapb.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>