priv->tx_hdr_len is set by the driver _after_ it called p54_init_common.
While this isn't much a problem for any PCI or ISL3887 cards/sticks,
because they don't need any extra header and therefore tx_hdr_len is
zero for them...
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>