When booting some versions of the UEFI shell, our driver binding
protocol's Supported() entry point is called at TPL_NOTIFY for no
discernible reason.  Attempting to raise to TPL_CALLBACK triggers an
immediate assertion failure in the firmware.

Since our Supported() method can run at any TPL, fix by simply not
attempting to raise the TPL within this method.

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