Adjust sllin configuration utility to specify external expected response length.

The setup of the length has meaning not only for sent frames
but even for received responses. The new kernel driver
code uses DLC value stored in the cache for received frames.
The LIN_CACHE_RESPONSE is used only for IDs which
should be sent by the kernel driver.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>