Bluetooth: Controller: LLCP: Fix handling of invalid DLE parameters

If peer sents invalid value in DLE request or response just ignore
those and keep using old values when calculating effective DLE.

This was affecting following qualification test cases:
LL/CON/PER/BI-10-C
LL/CON/PER/BI-11-C
LL/CON/PER/BI-12-C
LL/CON/CEN/BI-07-C
LL/CON/CEN/BI-08-C
LL/CON/CEN/BI-09-C

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>