net: esp{4,6}: get rid of struct esp_data

struct esp_data consists of a single pointer, vanishing the need for it
to be a structure. Fold the pointer into 'data' direcly, removing one
level of pointer indirection.

Signed-off-by: Mathias Krause <mathias.krause@secunet.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>