e1000e: fix call to do_div() to use u64 arg

We were using s64 for lat_ns (latency nano-second value) since in
our calculations a negative value could be a resultant.  For negative
values, we then assign lat_ns to be zero, so the value passed to
do_div() was never negative, but do_div() expects the argument type
to be u64, so do a cast to resolve a compile warning seen on
PowerPC.

CC: Yanjiang Jin <yanjiang.jin@windriver.com>
CC: Yanir Lubetkin <yanirx.lubetkin@intel.com>
Reported-by: Yanjiang Jin <yanjiang.jin@windriver.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>

drivers/net/ethernet/intel/e1000e/ich8lan.c

@@ -1015,7 +1015,7 @@ static s32 e1000_platform_pm_pch_lpt(struct e1000_hw *hw, bool link)
 		u16 max_snoop, max_nosnoop;
 		u16 max_ltr_enc;	/* max LTR latency encoded */
 		s64 lat_ns;	/* latency (ns) */
-		s64 value;
+		u64 value;
 		u32 rxa;
 
 		if (!hw->adapter->max_frame_size) {
@@ -1042,12 +1042,13 @@ static s32 e1000_platform_pm_pch_lpt(struct e1000_hw *hw, bool link)
 		 */
 		lat_ns = ((s64)rxa * 1024 -
 			  (2 * (s64)hw->adapter->max_frame_size)) * 8 * 1000;
-		if (lat_ns < 0)
-			lat_ns = 0;
-		else
-			do_div(lat_ns, speed);
-
+		if (lat_ns < 0) {
+			value = 0;
+		} else {
 			value = lat_ns;
+			do_div(value, speed);
+		}
+
 		while (value > PCI_LTR_VALUE_MASK) {
 			scale++;
 			value = DIV_ROUND_UP(value, (1 << 5));