bcm53xx: disable CONFIG_FW_LOADER_USER_HELPER_FALLBACK

There are many targets using user space scripts to generate firmware but
bcm53xx doesn't need this, so let's disable that kernel option.

This lets us avoid some scary-looking kernel warnings like:
brcmfmac 0001:04:00.0: Falling back to user helper
firmware brcm!brcmfmac43602-pcie.txt: firmware_loading_store: map pages failed

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>