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: NJiri Slaby <jirislaby@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>