mt76: mt7915: Fix build error

In file included from ./include/linux/firmware.h:6:0,
                 from drivers/net/wireless/mediatek/mt76/mt7915/mcu.c:4:
In function ‘__mt7915_mcu_msg_send’,
    inlined from ‘mt7915_mcu_send_message’ at drivers/net/wireless/mediatek/mt76/mt7915/mcu.c:370:6:
./include/linux/compiler.h:396:38: error: call to ‘__compiletime_assert_545’ declared with attribute error: BUILD_BUG_ON failed: cmd == MCU_EXT_CMD_EFUSE_ACCESS && mcu_txd->set_query != MCU_Q_QUERY
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
                                      ^
./include/linux/compiler.h:377:4: note: in definition of macro ‘__compiletime_assert’
    prefix ## suffix();    \
    ^~~~~~
./include/linux/compiler.h:396:2: note: in expansion of macro ‘_compiletime_assert’
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro ‘compiletime_assert’
 #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
                                     ^~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:50:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
  ^~~~~~~~~~~~~~~~
drivers/net/wireless/mediatek/mt76/mt7915/mcu.c:280:2: note: in expansion of macro ‘BUILD_BUG_ON’
  BUILD_BUG_ON(cmd == MCU_EXT_CMD_EFUSE_ACCESS &&
  ^~~~~~~~~~~~

BUILD_BUG_ON is meaningless here, chang it to WARN_ON.

Fixes: e57b7901

 ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200522034533.61716-1-yuehaibing@huawei.com