plugins: modem-manager: assume firmware is unsigned
plugins: modem-manager: assume firmware is unsigned

fwupd does not know if the firmware is signed or unsigned unless
the Quectel secureboot commands set this flag. Assume that the firmware
is unsigned by default, which is the case for most firmware unless they
have they support the secureboot AT commands. If that's the case, the
right flag will be set anyway.

fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd