efi: pstore: Return proper errors on UEFI failures

Right now efi-pstore either returns 0 (success) or -EIO; but we
do have a function to convert UEFI errors in different standard
error codes, helping to narrow down potential issues more accurately.

So, let's use this helper here.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>