serial: omap: stick to put_autosuspend

Everytime we're done using our TTY, we want
the pm timer to be reinitilized. By sticking
to pm_runtime_pm_autosuspend() we make sure
that this will always be the case.

The idea behind this patch is to make sure we
will always reinitialize the pm timer so that
we don't fall into a situation where pm_runtime_put()
expires right away (if timer was already about to
expire when we made the call to pm_runtime_put()).

While suspending right away wouldn't cause any
issues, reinitializing the pm timer can help us
avoiding unnecessary context save & restore
operations (which are somewhat expensive) if there's
another read/write/set_termios request coming right
after. IOW, we are trying to make sure UART is still
powered up while it's still under heavy usage.

Tested-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>