tpm_i2c_stm_st33: fix oops when i2c client is unavailable

When no i2c bus exists, user-space can cause an oops by triggering a
device probe through a message sent to an i2c "new_device" sysfs entry.
Adding a check for a NULL i2c client structure in the probe function
closes the hole.

This patch also fixes accessing the NULL client struct in the print
function call reporting the error.

Reported-by: Peter Hüwe <PeterHuewe@gmx.de>
Signed-off-by: Kent Yoder <key@linux.vnet.ibm.com>