ixgbe: Convert IXGBE_DESC_UNUSED from macro to static inline function

This change is a minor cleanup that converts the IXGBE_DESC_UNUSED macro
into a static inline function just for the case of the code being a bit
cleaner.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
diff --git a/drivers/net/ixgbe/ixgbe.h b/drivers/net/ixgbe/ixgbe.h
index d6bfb2f..b1d4b02 100644
--- a/drivers/net/ixgbe/ixgbe.h
+++ b/drivers/net/ixgbe/ixgbe.h
@@ -306,9 +306,13 @@
 	((_eitr) ? (1000000000 / ((_eitr) * 256)) : 8)
 #define EITR_REG_TO_INTS_PER_SEC EITR_INTS_PER_SEC_TO_REG
 
-#define IXGBE_DESC_UNUSED(R) \
-	((((R)->next_to_clean > (R)->next_to_use) ? 0 : (R)->count) + \
-	(R)->next_to_clean - (R)->next_to_use - 1)
+static inline u16 ixgbe_desc_unused(struct ixgbe_ring *ring)
+{
+	u16 ntc = ring->next_to_clean;
+	u16 ntu = ring->next_to_use;
+
+	return ((ntc > ntu) ? 0 : ring->count) + ntc - ntu - 1;
+}
 
 #define IXGBE_RX_DESC_ADV(R, i)	    \
 	(&(((union ixgbe_adv_rx_desc *)((R)->desc))[i]))