USB: serial: usb_wwan: fix TIOCGSERIAL implementation

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 port parameter is used to set the I/O port and does not make any
sense to use for USB serial devices.

The baud_base parameter 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.

Fix the usb_wwan TIOCGSERIAL implementation by dropping its custom
interpretation of the unused port and baud_base fields, which were set
to the port index and current line speed, respectively.

Fixes: 02303f73 ("usb-wwan: implement TIOCGSERIAL and TIOCSSERIAL to avoid blocking close(2)")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>