mt76: mt7615: Fix build with older compilers

Some compilers (tested with 4.8.5 from CentOS 7) fail properly process
FIELD_GET inside an inline function, which ends up in a BUILD_BUG_ON.
Convert inline function to a macro.

Fixes commit bf92e768 ("mt76: mt7615: add support for per-chain
signal strength reporting")
Reported in https://lkml.org/lkml/2019/9/21/146Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Pablo Greco <pgreco@centosproject.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7615/mac.c

@@ -13,10 +13,7 @@
 #include "../dma.h"
 #include "mac.h"
 
-static inline s8 to_rssi(u32 field, u32 rxv)
-{
-	return (FIELD_GET(field, rxv) - 220) / 2;
-}
+#define to_rssi(field, rxv)		((FIELD_GET(field, rxv) - 220) / 2)
 
 static struct mt76_wcid *mt7615_rx_get_wcid(struct mt7615_dev *dev,
 					    u8 idx, bool unicast)