drivers: modem: sara-r4: Improve offload_sendmsg() support

Previously, msghdr was handled by forwarding each msg_iov to
offload_sendto() one by one.
This is not optimal, as msg_iov payload might have been sent
simultaneously which would be faster.
With this commit, send_socket_data() expect payload to be formated as
struct msghdr which can be used directly by both offload_sendto() and
offload_sendmsg().

Signed-off-by: Xavier Chapron <xavier.chapron@stimio.fr>