qcacmn: dp: Replace fallthrough comment with attribute

-Wimplicit-fallthrough is being enabled by default. Some compilers
such as clang require the attribute instead of just a fallthrough comment.

Change-Id: I443da8d7f5e1771dceb3386c4458b0da6a5e9476
CRs-Fixed: 3218236

dp/wifi3.0/be/dp_be.h

@@ -719,7 +719,7 @@ _dp_srng_test_and_update_nf_params(struct dp_soc *soc,
 		switch (near_full_level) {
 		case DP_SRNG_THRESH_CRITICAL:
 			/* Currently not doing anything special here */
-			/* fall through */
+			fallthrough;
 		case DP_SRNG_THRESH_NEAR_FULL:
 			ring_near_full = 1;
 			*max_reap_limit *= DP_SRNG_PER_LOOP_NF_REAP_MULTIPLIER;