serial: 8250: 8250_omap: Fix throttle to call stop_rx()

Call stop_rx() to halt reception when throttle is requested. Update
unthrottle callback to restart reception.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20200319103230.16867-3-vigneshr@ti.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>