fix for integer overflow in hd6457[02] driver code

Hi,

The attached patch fixes an integer overflow in drivers for N2, C101,
PCI200SYN WAN cards (brv * port->settings.clock_rate overflowed at
requested clock rate of 8*1024*1024 bps, problem noted by Nagaraj Kanniah).

Please apply to 2.4 and 2.6 kernel trees. Thanks.
-- 
Krzysztof Halasa