amdtee: Use pr_fmt for messages

amd-tee inconsistently uses the word amd-tee in error messages.

Add a `pr_fmt()` define and remove the hardcoded use of amd-tee.
Also, remove the unnecessary driver init successful message because
there will be a message if failed and a user can assume it was successful
otherwise.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>