hdlcdrv: Fix CRC calculation.

This is a trivial patch against the hdlcdrv module that fixes its CRC
calculation. The finished CRC was overwriting the first two bytes of
each packet rather than being appended to the end.

I've tested this with 2.6.8 and 2.6.10-rc1, but hdlcdrv hasn't changed
much recently so it should work with many other kernel versions.

Signed-off-by: Micah Dowty <micah@navi.cx>
Acked-by: Thomas Sailer <t.sailer@alumni.ethz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>