net: convert SMSC USB net drivers to hw_features

There's a race (not fixed here) in smsc75xx in setting RFE_CTL that's not
properly handled via rfe_ctl_lock. Spinlock is not a good tool here, as
this has to wait for URB completion (or maybe just submission) after issuing
register write request. Otherwise, the rfe_ctl might be changed just after
spin_unlock() and device left programmed with other value.

smsc95xx has increased hard_header_len for the case of TX checksumming.

smsc75xx is fixed to advertise IP+IPV6_CSUM instead of HW_CSUM as it does
not use csum_start/csum_offset.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>