serial: omap: fix sequence of pm_runtime_* calls.

pm_runtime_enable() needs to be invoked before
pm_runtime_use_autosuspend(), and
pm_runtime_set_autosuspend_delay() functions.

Tested-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Ruchika Kharwar <ruchika@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>