[efi] Allow for 64-bit EFI_STATUS codes

On a 64-bit build, EFI_STATUS codes are 64-bit quantities, with the
"error/warning" bit located in bit 63.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Allow driver to be unloaded

Signed-off-by: Michael Brown <mcb30@ipxe.org>