mt76: mt7615: fix mt7615 firmware path definitions

mt7615 patch/n9/cr4 firmwares are available in mediatek folder in
linux-firmware repository. Because of this mt7615 won't work on regular
distributions like Ubuntu. Fix path definitions.  Moreover remove useless
firmware name pointers and use definitions directly

Fixes: 04b8e659 ("mt76: add mac80211 driver for MT7615 PCIe-based chipsets")
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>