USB: ir-usb: allow custom bulk buffer size without reallocation

Use usb_serial_driver bulk_in_size and bulk_out_size to make sure
buffers of appropriate sizes are allocated in the first place rather than
reallocating them at every open.

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