watchdog: Reduce message verbosity

Code is emitting the following error message during boot on systems
without PMU hardware support while probing NMI capability.

 NMI watchdog: Perf event create on CPU 0 failed with -2

This error is emitted as the perf subsystem returns -ENOENT due to lack of
PMUs in the system.

It is followed by the warning that NMI watchdog is disabled:

  NMI watchdog: Perf NMI watchdog permanently disabled

While NMI disabled information is useful for ordinary users, seeing a PERF
event create failed with error code -2 is not.

Reduce the message severity to debug so that if debugging is still possible
in case the error code returned by perf is required for analysis.

Signed-off-by: Sinan Kaya <okaya@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Don Zickus <dzickus@redhat.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=599368
Link: https://lkml.kernel.org/r/20180803060943.2643-1-okaya@kernel.org