wifi: esp_at: claim net_context in rx

Claim the net_context mutext associated with a socket before claiming
the socket mutex. The receive callback claims the net_context mutex
internally, which will now always succeed immediately.

The TX path claims the net_context mutex before the socket mutex, and if
we don't use the same order, we can end up in a deadlock.

Fixes #43470.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>