staging: ozwpan: Fix hrtimer wreckage

oz_timer_add() modifies the expiry value of an active timer, which
results in data corruption.

Use hrtimer_start() and remove the silly conditional.

While at it use the proper helper function to convert milliseconds to
ktime.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shigekatsu Tateno <shigekatsu.tateno@atmel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: devel@driverdev.osuosl.org
Link: http://lkml.kernel.org/r/20150413210035.357448657@linutronix.de