efi_loader: do not cast return value in EFI_EXIT

UEFI API functions have different return types.
Some return a value of type EFI_STATUS other don't.

We therefore should not cast the return value of EFI_EXIT
to another type than the expression passed to EFI_EXIT.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>