efi: pstore: Follow convention for the efi-pstore backend name

For some reason, the efi-pstore backend name (exposed through the
pstore infrastructure) is hardcoded as "efi", whereas all the other
backends follow a kind of convention in using the module name.

Let's do it here as well, to make user's life easier (they might
use this info for unloading the module backend, for example).

Cc: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221006224212.569555-8-gpiccoli@igalia.com