moxa, fix and optimise empty timer

don't wait and delete empty timer in empty timer function. Also fire next
empty timer at rounded jiffies to save power.
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>