Xen: fix braces coding style issue in xenbus_probe.h

This is a patch to the xenbus_probe.h file that fixed up braces errors found by the checkpatch.pl tools.

Signed-off-by: Ruslan Pisarev <ruslan@rpisarev.org.ua>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>