Bluetooth: fix MTU L2CAP configuration parameter

When receiving L2CAP negative configuration response with respect
to MTU parameter we modify wrong field. MTU here means proposed
value of MTU that the remote device intends to transmit. So for local
L2CAP socket it is pi->imtu.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@nokia.com>
Acked-by: Ville Tervo <ville.tervo@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>