tlan: Fix two regressions introduced by 64bit conversion.

Two regressions were introduced by the recent tlan: 64bit conversion
commit (93e16847).  The first in
TLan_GetSKB caused a NULL pointer dereference.  With the second causing
the link to fail to come up.

Signed-off-by: Robert Fitzsimons <robfitz@273k.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>