net: l2: ppp: possibility to have a custom MRU/MTU

Reading a set MTU from the used net_if when starting LCP.
This enables also other custom MTU/MRU to be set for the link than
the default CONFIG_NET_PPP_MTU_MRU (set by a ppp driver during
initialization).

Signed-off-by: Jani Hirsimäki <jani.hirsimaki@nordicsemi.no>