drivers/wifi/winc1500: Fix build warnings with 64bit targets

Fix a set of warnings like:
drivers/wifi/winc1500/wifi_winc1500.c:332:25: error: cast from pointer
to integer of different size [-Werror=pointer-to-int-cast]
  332 |         SOCKET socket = (int)context->offload_context;

when int and void* do not have the same size, by casting the content
of context->offload_context to intptr_t which is whichever integer
size matches the pointer size.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>