I'm no expert either and I have not done kernel development, but I've done some embedded stuff in C and I think this is not unreasonable. brcmf_reg_read is only called in one place and the call chain is straightforward (starts in pcie.c brcmf_pcie_attach). Its always initialized by device_get_softc (https://man.freebsd.org/cgi/man.cgi?query=device_get_softc&s...) so as long as the device is attached its initialized. Likely something fails much earlier if it is not attached. I think this is pretty typical for low-level C, it would definitely not be idiomatic for every function in this call chain to check if sc was initialized - I don't know if there is a need to check it after calling device_get_softc but that would probably be reasonable just so people reviewing it don't have to check.

Some application code bases I've worked in would have asserts in every function since they get removed in release builds but I don't know that debug builds are even a thing in kernel devices.