mt76: Fix queue ID variable types after mcu queue split

Clang warns in both mt7615 and mt7915:

drivers/net/wireless/mediatek/mt76/mt7915/mcu.c:271:9: warning: implicit
conversion from enumeration type 'enum mt76_mcuq_id' to different
enumeration type 'enum mt76_txq_id' [-Wenum-conversion]
                txq = MT_MCUQ_FWDL;
                    ~ ^~~~~~~~~~~~
drivers/net/wireless/mediatek/mt76/mt7915/mcu.c:278:9: warning: implicit
conversion from enumeration type 'enum mt76_mcuq_id' to different
enumeration type 'enum mt76_txq_id' [-Wenum-conversion]
                txq = MT_MCUQ_WA;
                    ~ ^~~~~~~~~~
drivers/net/wireless/mediatek/mt76/mt7915/mcu.c:282:9: warning: implicit
conversion from enumeration type 'enum mt76_mcuq_id' to different
enumeration type 'enum mt76_txq_id' [-Wenum-conversion]
                txq = MT_MCUQ_WM;
                    ~ ^~~~~~~~~~
3 warnings generated.

drivers/net/wireless/mediatek/mt76/mt7615/mcu.c:238:9: warning: implicit
conversion from enumeration type 'enum mt76_mcuq_id' to different
enumeration type 'enum mt76_txq_id' [-Wenum-conversion]
                qid = MT_MCUQ_WM;
                    ~ ^~~~~~~~~~
drivers/net/wireless/mediatek/mt76/mt7615/mcu.c:240:9: warning: implicit
conversion from enumeration type 'enum mt76_mcuq_id' to different
enumeration type 'enum mt76_txq_id' [-Wenum-conversion]
                qid = MT_MCUQ_FWDL;
                    ~ ^~~~~~~~~~~~
2 warnings generated.

Use the proper type for the queue ID variables to fix these warnings.
Additionally, rename the txq variable in mt7915_mcu_send_message to be
more neutral like mt7615_mcu_send_message.

Fixes: e637763b ("mt76: move mcu queues to mt76_dev q_mcu array")
Link: https://github.com/ClangBuiltLinux/linux/issues/1229


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201229211548.1348077-1-natechancellor@gmail.com