um: rename set_signals() to um_set_signals()

Rename set_signals() as there's at least one driver that
uses the same name and can now be built on UM due to PCI
support, and thus we can get symbol conflicts.

Also rename set_signals_trace() to be consistent.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 68f5d3f3

 ("um: add PCI over virtio emulation driver")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>