serial: 8250: Use BIT(x) for UART_{CAP,BUG}_*

BIT(x) improves readability and safety with respect to shifts.

Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20210520021334.497341-3-andrew@aj.id.au


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