efi_loader: update the error message of TCG protocol installation

"Unable to find TPMv2 device" doesn't explain much with regards to the
error origin. Update it to match what we have in the RNG protocol
installation.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>