time: Refactor usecs_to_jiffies

Refactor the usecs_to_jiffies conditional code part in time.c and
jiffies.h putting it into conditional functions rather than #ifdefs
to improve readability. This is analogous to the msecs_to_jiffies()
cleanup in commit ca42aaf0 ("time: Refactor msecs_to_jiffies")

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Joe Perches <joe@perches.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Andrew Hunter <ahh@google.com>
Cc: Paul Turner <pjt@google.com>
Cc: Michal Marek <mmarek@suse.cz>
Link: http://lkml.kernel.org/r/1432832996-12129-1-git-send-email-hofrat@osadl.org


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>