e1000: FIX: fix wrong txdctl threshold bitmasks

Threshold bitmasks for prefetch, host and writeback were clearing
bits that they were not supposed to. The leftmost 2 bits in the byte
for each threshold are reserved.
Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>

drivers/net/e1000/e1000_hw.h

@@ -1961,9 +1961,9 @@ struct e1000_hw {
 #define E1000_RXDCTL_GRAN    0x01000000 /* RXDCTL Granularity */
 
 /* Transmit Descriptor Control */
-#define E1000_TXDCTL_PTHRESH 0x000000FF /* TXDCTL Prefetch Threshold */
-#define E1000_TXDCTL_HTHRESH 0x0000FF00 /* TXDCTL Host Threshold */
-#define E1000_TXDCTL_WTHRESH 0x00FF0000 /* TXDCTL Writeback Threshold */
+#define E1000_TXDCTL_PTHRESH 0x0000003F /* TXDCTL Prefetch Threshold */
+#define E1000_TXDCTL_HTHRESH 0x00003F00 /* TXDCTL Host Threshold */
+#define E1000_TXDCTL_WTHRESH 0x003F0000 /* TXDCTL Writeback Threshold */
 #define E1000_TXDCTL_GRAN    0x01000000 /* TXDCTL Granularity */
 #define E1000_TXDCTL_LWTHRESH 0xFE000000 /* TXDCTL Low Threshold */
 #define E1000_TXDCTL_FULL_TX_DESC_WB 0x01010000 /* GRAN=1, WTHRESH=1 */