cmd: gpio: Correct do_gpio() return value

Use the correct return value in function do_gpio() and update
commands documentation with the return values from command_ret_t enum.

CMD_RET_SUCCESS is returned on command success and CMD_RET_FAILURE is
returned on command failure.

The command was returning the pin value, which caused confusion when
debugging (#define DEBUG).

Signed-off-by: Luka Kovacic <luka.kovacic@sartura.hr>
Tested-by: Robert Marko <robert.marko@sartura.hr>