tty: 8250, use ms_to_ktime

This really eliminates multiplications from the assembly. I would have
thought they are optimized by inlining ktime_set, but not on x86_64 with
gcc 10.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-11-jslaby@suse.cz


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