hrtimer: Clean up 'enum hrtimer_mode'

hrtimer: Clean up 'enum hrtimer_mode'

It's not obvious that the HRTIMER_MODE variants are bit combinations,
because all modes are hard coded constants currently.

Change it so the bit meanings are clear; and use the symbols for creating
modes which combine bits.

While at it get rid of the ugly tail comments as well.

Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: keescook@chromium.org
Link: http://lkml.kernel.org/r/20171221104205.7269-8-anna-maria@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>