net: cnss: add config flag for cnss common compilation

The cnss common kernel module export symbol to WLAN host
driver for ROME based wlan module. Add kernel config
flag CONFIG_CNSS to make this module compilation only
for ROME based target.

Change-Id: Ia352f0b561f3ec5a53f0d48fd849e43caf4e44a2
Signed-off-by: Sarada Prasanna Garnayak <sgarna@codeaurora.org>