lis3: remove automatic shutdown of the device

After measurement on my laptop, it seems that turning off the device does
not bring any energy saving (within 0.1W precision).  So let's keep the
device always on.  It simplifies the code, and it avoids the problem of
reading a wrong value sometimes just after turning the device on.

Moreover, since commit ef2cfc79 had been
too zealous, the device was actually never turned off anyway.  This patch
also restores the damages done by this commit concerning the
initialisation/poweroff.

Also do more clean up with the usage of the lis3_dev global variable.

Signed-off-by: Eric Piel <eric.piel@tremplin-utc.net>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>