wcnss: Fix for Static analysis issues

In wcnssctrl_rx_handler value of len could become negative
after substracting sizeof(struct smd_msg_hdr) from len.
There is no check to verify if value of len is less than
sizeof(struct smd_msg_hdr).

Adds proper check to verify if value of len is less than
sizeof(struct smd_msg_hdr).

Change-Id: I8a3bc9eb63e1b232023444bb8258bcdaccfa1c34
CRs-Fixed: 633938
Signed-off-by: Abhishek Singh <absingh@codeaurora.org>