serial: omap: make sure to put() on poll_get_char

if we would reach serial_omap_get_char() while
Data Ready bit isn't set, we would return from
it without kicking our pm timer. This would mean
we would, eventually, have an unbalanced
pm_runtime_get on our device which would prevent
it from ever sleeping again.

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