[efi] Raise TPL during driver unload entry point

[efi] Raise TPL during driver unload entry point

The efi_unload() function is currently missing the calls to raise and
restore the TPL.  This has the side effect of causing iPXE to return
from the driver unload entry point at TPL_CALLBACK, which will cause
unexpected behaviour (typically a system lockup) shortly afterwards.

Fix by adding the missing calls to raise and restore the TPL.

Debugged-by: Petr Borsodi <petr.borsodi@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>