That's a possibility that I thought of too, but considering that each object is separately hashed, just getting random bytes into some random firmware doesn't help all that much.
Basically, in addition to that, you'd have to:
1) Insert the payload into the SAME firmware file (a different file will have its own SHA1, so it's gotta be the same file).
2) Somehow, convince Linus to accept a patch that loads that firmware file, either for your specific target if you have one (not out of the realm of possibility), or for a rather large segment of the kernel running population (rather more difficult, given kernel modularity, but it depends on just how large a segment you want, if everyone running a specific NIC or graphics chip is enough, it's not too difficult, but if you want nearly everyone running a Linux kernel, it's VERY difficult indeed!).
Even if that occurs, you then have to wait until it's actually deployed on your target, with some targets not updating for years, hoping it's not caught in the mean time.
So successful attack via firmware is indeed theoretically possible, but still not particularly simple in practice. There's almost certainly faster and less resource intensive compromise methods, so it's unlikely to be used in practice.