efi/libstub: Upgrade ignored dtb= argument message to error

Use efi_err if we ignore a command-line dtb= argument, so that it shows
up even on a quiet boot.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200430182843.2510180-8-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>