serial: 8250_aspeed_vuart: factor out aspeed_vuart_{read, write}b() helper functions

This is a small prepatory step for changing the way this driver does
its I/O accesses.

Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Link: https://lore.kernel.org/r/20210510014231.647-2-zev@bewilderbeest.net


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