tmr: softwd can't be disabled in accordance to manual

[Moritz Wilhelmy]

The manual mentions tmr.softwd(-1) (or any other negative number) as a means to disarm the watchdog, however the C function claims this is an invalid time.

nodemcu-firmware/app/modules/tmr.c

Line 277 in 4f67927

 luaL_argcheck(L, t>0 , 2, "invalid time"); 

[Gregor Hartmann]

Has been fixed with #3326 and is available on dev builds for now.