efi_loader: fix DisconnectController() for sole child

If ChildHandle indicates the sole child of the driver, disconnect the
driver.

This fixes the test results for UEFI SCT 2.6 A
sub-tests 5.1.3.12.43, 5.1.3.12.44, 5.1.3.12.45.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>