net: l2: ppp: config: fix for max terminate-req transmissions

CONFIG_NET_L2_PPP_MAX_TERMINATE_REQ_RETRANSMITS
was not having any impact and
CONFIG_NET_L2_PPP_MAX_CONFIGURE_REQ_RETRANSMITS
was used incorrectly instead for terminate().

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