add back ethercrc.

Reference: /n/atom/patch/applied/ethercrcredux
Date: Thu May  1 18:19:30 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/devether.c	Thu May  1 18:19:25 2014
+++ /sys/src/nix/k10/devether.c	Thu May  1 18:19:25 2014
@@ -448,6 +448,26 @@
 	}
 }
 
+#define POLY 0xedb88320
+
+/* really slow 32 bit crc for ethers */
+u32int
+ethercrc(uchar *p, int len)
+{
+	int i, j;
+	u32int crc, b;
+
+	crc = 0xffffffff;
+	for(i = 0; i < len; i++){
+		b = *p++;
+		for(j = 0; j < 8; j++){
+			crc = (crc>>1) ^ (((crc^b) & 1) ? POLY : 0);
+			b >>= 1;
+		}
+	}
+	return crc;
+}
+
 Dev etherdevtab = {
 	'l',
 	"ether",