thunderbolt: Fix -Wrestrict warning

gcc-11 warns when building with W=1:

drivers/thunderbolt/xdomain.c: In function 'modalias_show':
drivers/thunderbolt/xdomain.c:733:16: error: 'sprintf' argument 3 overlaps destination object 'buf' [-Werror=restrict]
  733 |         return sprintf(buf, "%s\n", buf);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~
drivers/thunderbolt/xdomain.c:727:36: note: destination object referenced by 'restrict'-qualified argument 1 was declared here
  727 |                              char *buf)
      |                              ~~~~~~^~~

There is no need for the sprintf() here when a strcat() does
the same thing.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>