nimble/ble_gap: remove extraneous parentheses

Fix build warning generated with llvm based compilers on equality comparison with extraneous parentheses.
diff --git a/nimble/host/src/ble_gap.c b/nimble/host/src/ble_gap.c
index b264efd..44d5770 100644
--- a/nimble/host/src/ble_gap.c
+++ b/nimble/host/src/ble_gap.c
@@ -3720,7 +3720,7 @@
         /* If there is an active instance or periodic adv instance,
          * Don't send the command
          * */
-        if ((ble_gap_slave[instance].op == BLE_GAP_OP_S_ADV)) {
+        if (ble_gap_slave[instance].op == BLE_GAP_OP_S_ADV) {
             ble_hs_unlock();
             return BLE_HS_EBUSY;
         }