tpm/tpm_ppi: Check return value of acpi_get_name

If
 status = acpi_get_name(handle, ACPI_FULL_PATHNAME, &buffer);
fails for whatever reason and does not return AE_OK
 if (strstr(buffer.pointer, context) != NULL) {
does dereference a null pointer.

-> Check the return value and return the status to the caller

Found by coverity
Cc: stable@vger.kernel.org
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>