kern/file: Fix error handling in grub_file_open()

kern/file: Fix error handling in grub_file_open()

grub_file_open() calls grub_file_get_device_name(), but doesn't check
the return. Instead, it checks if grub_errno is set.

However, nothing initialises grub_errno here when grub_file_open()
starts. This means that trying to open one file that doesn't exist and
then trying to open another file that does will (incorrectly) also
fail to open that second file.

Let's fix that.

Signed-off-by: Steve McIntyre <steve@einval.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>