firmware: Fix inverted FIRMWARE_GET_NOWARN logic

[guest-seuros]

Summary

The try_binary_file() function has inverted logic for the FIRMWARE_GET_NOWARN flag.
When the flag is set (meaning "don't warn"), the code sets warn=true and makes noise anyway.

Rename the variable to nowarn and invert the check to correctly suppress warnings when FIRMWARE_GET_NOWARN is set.

[kevans]

sys/kern/subr_firmware.c

341

I think this would read a bit better after it was still named warn and we just fixed the logic above and here; double-negatives are kind of not great

[guest-seuros]

Address review feedback: keep warn variable, invert at assignment instead of double-negative