ATM-nicstar: Improve another size determination in ns_init_card()

Replace the specification of a data structure by a reference for a field
in a local variable as the parameter for the operator "sizeof" to make
the corresponding size determination a bit safer according to
the Linux coding style convention.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>