[efi] Drop to external TPL for calls to ConnectController()

[efi] Drop to external TPL for calls to ConnectController()

There is nothing in the current versions of the UEFI specification
that limits the TPL at which we may call ConnectController() or
DisconnectController().  However, at least some platforms (observed
with a Lenovo ThinkPad T14s Gen 5) will occasionally and unpredictably
lock up before returning from ConnectController() if called at a TPL
higher than TPL_APPLICATION.

Work around whatever defect is present on these systems by dropping to
the current external TPL for all calls to ConnectController() or
DisconnectController().

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