Clean up get_seqnum() use for IPN

Clean up get_seqnum() use for IPN

Some driver wrappers may implement this by writing eight octets even
though IPN is only six octets. Use a separate WPA_KEY_RSC_LEN (8) octet
buffer in the call to make sure there is enough buffer room available
for the full returned value and then copy it to IPN field.

The previous implementation used the following igtk field as the extra
buffer and then initialized that field afterwards, so this change does
not fix any real issue in behavior, but it is cleaner to use an explicit
buffer of the maximum length for get_seqnum().

Signed-hostap: Jouni Malinen <j@w1.fi>