lwip: Rename `netif_context_t` variables

Variables named `netif_context_t` cause compiler
warnings to be generated due to the exact same name
being used for a custom user type.
As these variables are local, they are renamed
to `context` keeping in line with other local instances
of the `netif_context_t` type.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>