usb: pl2303: do not round to the next nearest standard baud rate for the divisor based baud rate encoding method

In opposition to the direct baud rate encoding method, the divisor
based method is not limited to a fixed set of standard baud rates.
Hence, there is no need to round to the next nearest standard value.
Reported-by: Mastro Gippo <gipmad@gmail.com>
Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Reinhard Max <max@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>