mt76: mt7915: fix unused 'mode' variable

clang points out a possible corner case in the mt7915_tm_set_tx_cont()
function if called with invalid arguments:

drivers/net/wireless/mediatek/mt76/mt7915/testmode.c:593:2: warning: variable 'mode' is used uninitialized whenever switch default is taken [-Wsometimes-uninitialized]
        default:
        ^~~~~~~
drivers/net/wireless/mediatek/mt76/mt7915/testmode.c:597:13: note: uninitialized use occurs here
        rateval =  mode << 6 | rate_idx;
                   ^~~~
drivers/net/wireless/mediatek/mt76/mt7915/testmode.c:506:37: note: initialize the variable 'mode' to silence this warning
        u8 rate_idx = td->tx_rate_idx, mode;
                                           ^

Change it to return an error instead of continuing with invalid data
here.

Fixes: 3f0caa3c ("mt76: mt7915: add support for continuous tx in testmode")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt76/mt7915/testmode.c

@@ -543,7 +543,7 @@ mt7915_tm_set_tx_cont(struct mt7915_phy *phy, bool en)
 		tx_cont->bw = CMD_CBW_20MHZ;
 		break;
 	default:
-		break;
+		return -EINVAL;
 	}
 
 	if (!en) {
@@ -591,7 +591,7 @@ mt7915_tm_set_tx_cont(struct mt7915_phy *phy, bool en)
 		mode = MT_PHY_TYPE_HE_MU;
 		break;
 	default:
-		break;
+		return -EINVAL;
 	}
 
 	rateval =  mode << 6 | rate_idx;