bnxt_en: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of just letting the code
fall through to the next case.

Link: KSPP#115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Author: GustavoARSilva

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -2183,6 +2183,7 @@ static int bnxt_hwrm_handler(struct bnxt *bp, struct tx_cmp *txcmp)
 	case CMPL_BASE_TYPE_HWRM_ASYNC_EVENT:
 		bnxt_async_event_process(bp,
 					 (struct hwrm_async_event_cmpl *)txcmp);
+		break;
 
 	default:
 		break;