serial: 8250_omap: fix a timeout loop condition

This loop ends on -1 so the error message will never be printed.

Fixes: 4bcf59a5 ("serial: 8250: 8250_omap: Account for data in flight during DMA teardown")
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YIpd+kOpXKMpEXPf@mwanda


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