AP_SerialManager: allow for permanent passthrough

[tridge]

it is sometimes useful to setup a serial passthrough as permanent, this allows for that by setting SERIAL_PASSTIMO to -1

[andyp1per]

Although I think this is useful there is some danger - I have sometimes made my flight controllers inaccessible and the timeout has saved my bacon.

[tridge]

I have sometimes made my flight controllers inaccessible and the timeout has saved my bacon.

so don't enable the option if you don't want it permanent!
you can always install a different vehicle fw to reset
for me this was extremely useful in developing the driver for this PR:
#30213

[IamPete1]

LGTM, can see why it might be useful in some cases.