efi_loader: call __efi_exit_check in efi_exit

The calls to __efi_entry_check and __efi_exit_check have to match.
If DEBUG is defined, panic() will be called otherwise.

If debugging is activated some Travis CI builds fail due to an
assertion in EFI_CALL without the patch.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>