USB: serial: make bulk_out_size a lower limit

Drivers are allowed to override the default bulk-out buffer size
(endpoint maximum packet size) in order to increase throughput, but it
does not make much sense to allow buffers smaller than the default.

Note that this is already how bulk_in_size is defined.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>