tty: serial: 8250: omap: synchronize rx_running

The rx_running flag should show if DMA is currently active. However
there is a window between when the flag is set/cleared and when
the DMA is started/stopped. Because the flag is queried from both
hard and soft irq contexts, the driver can make incorrect
decisions and do things like start a DMA transfer using a buffer
that is already setup to be used for a DMA transfer.

This patch adds a spinlock to synchronize the rx_running flag and
close the above mentioned window.

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