usb: pl2303: remove 500000 baud from the list of standard baud rates

Commit 0c967e7e "USB: serial: pl2303 works at 500kbps" added 500000
baud to the list of supported standard baud rates.
But the reason why the driver works with this baud rate is, that since
commit 8d48fdf6

 "USB: PL2303: correctly handle baudrates above 115200"
a second (divisor based) baud rate encoding method is used for values
above 115200 baud, which is not limited to a fixed set of standard baud
rates.

Remove the 500000 baud value from the list of standard baud rates
again, because this list is only used with the direct baud rate
encoding method and 500000 baud is not supported with this method.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>