net: l2: ppp: fix MRU when CONFIG_NET_L2_PPP_OPTION_MRU=n

When support for custom MRU/MTU was added with
CONFIG_NET_L2_PPP_OPTION_MRU=y, code flow with
CONFIG_NET_L2_PPP_OPTION_MRU=n has been broken due to lack of valid
'ctx->lcp.my_options.mru' initialization and its use (with value 0) in
the implementation.

Initialize 'ctx->lcp.my_options.mru' unconditionally in lcp_init(), so
that PPP works fine with CONFIG_NET_L2_PPP_OPTION_MRU=n.

Fixes: 8a51a79d ("net: l2: ppp: possibility to have a custom
  MRU/MTU")

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>