When an UART is opened that still has .throttled set from a previous
open, the RX interrupt is enabled but the irq handler doesn't consider
it. This easily results in a stuck irq with the effect to occupy the CPU
in a tight loop.

So reset the throttle state in .startup() to ensure that RX irqs are
handled.

Fixes: d1ec8a2e ("serial: stm32: update throttle and unthrottle ops for dma mode")
Cc: stable@vger.kernel.org
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/a784f80d3414f7db723b2ec66efc56e1ad666cbf.1713344161.git.u.kleine-koenig@pengutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>