USB: pl2303: increase the bulk-out buffer size to 256 byte

Increase the bulk-out buffer size from 64 to 256 byte.

This gives a significant increase in throughput already at 1Mbaud as well
as lowered CPU usage. The buffer is big enough to keep up also at 3Mbaud
(128b would not suffice).

        64b         256b
921k:   640 KB/s    870 KB/s
3M:     640 KB/s    2520 KB/s

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>