(Sorry, I miss clicked and deleted the wrong comment. Here is it but in wrong order.)
Actually, if the device has no possibility to do firmware upgrades, then that is in compliance with the the license.
GPL has an exception which say that the requirement "does not apply if neither you nor any third party retains the ability to install modified object code on the User Product (for example, the work has been installed in ROM)".
The goal is that "modified object code is in no case prevented or interfered with solely because modification has been made". It is targeting the intentional anti-consumer and anti-property design in which the manufactorer retain control of the device, after sale. If you don't intentionally prevent others from modifying the device, then the product is unlikely to be effected by the license requirement.
(The quoted parts are directly taken from the GPL license text, section 6).
