Bluetooth: Only enable data length extensions when needed

We should disable the feature when it is not needed to
save flash and RAM.
There is no point in enabling data length extensions
when the maximum packet size used is always smaller or equal
to 27 bytes. Then data length updates would only use
parameters (octets=27, time=T) where T is some supported value
which would not improve throughput or power consumption.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>