staging: greybus: uart: clean up TIOCGSERIAL

staging: greybus: uart: clean up TIOCGSERIAL

TIOCSSERIAL is a horrid, underspecified, legacy interface which for most
serial devices is only useful for setting the close_delay and
closing_wait parameters.

The xmit_fifo_size parameter could be used to set the hardware transmit
fifo size of a legacy UART when it could not be detected, but the
interface is limited to eight bits and should be left unset when not
used.

Similarly, baud_base could be used to set the UART base clock when it
could not be detected but might as well be left unset when it is not
known.

The type parameter could be used to set the UART type, but is
better left unspecified (type unknown) when it isn't used.

Note that some applications have historically expected TIOCGSERIAL to be
implemented, but judging from the Debian sources, the port type not
being PORT_UNKNOWN is only used to check for the existence of legacy
serial ports (ttySn). Notably USB serial drivers like ftdi_sio have been
using PORT_UNKNOWN for twenty years without any problems.

Drop the bogus values provided by the greybus implementation.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210407102334.32361-8-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>