[SERIAL] Make uart_port flags a bitwise type

Same reasoning as commit 747c8a55
but this time we're making uart_port flags a bitwise type - not
all of these flags correspond with the old ASYNC_ flags, so there
is the possibility for bugs if the wrong ASYNC_* constants are
used.  Always use UPF_* constants for uart_port->flags.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>