brcmfmac: Switch to appropriate helper to load EFI variable contents

Avoid abusing the efivar layer by invoking it with locally constructed
efivar_entry instances, and instead, just call the EFI routines directly
if available.

Acked-by: Kalle Valo <kvalo@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>