this card is based on 16750 UART, modify sio(4) a bit to ignore 16750-specific
7th bit of MCR when probing card. This allows card to be detected and attached
as 16550A-compatible device. More work needs to be done in order to enable
nice 16750-specific features such as larger fifo buffer and higher speeds.
Sponsored by: IC Book Labs
MFC after: 2 weeks