efi_loader: correct UninstallProtocolInterface()

efi_loader: correct UninstallProtocolInterface()

When uninstalling a protocol the following steps are needed:
* request all drivers to disconnect
* close protocol for all non-drivers
* check if any open instance of the protocol exists on the handle and
  return EFI_ACCESS_DENIED in this case
* remove the protocol interface

By tort we tested for remaining open protocol instances already after
requesting drivers to disconnect.

With this correction the UEFI SCT II tests for UninstallProtocolInterface()
and ReinstallProtocolInterface are passed.

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