serial: 8250: move rx_running out of the bitfield

That bitfield is modified by read + or + write operation. If someone
sets any of the other two bits it might render the lock useless.

While at it, remove other bitfields as well to avoid more such
errors.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>