8139cp: properly support change of MTU values [v2]

The 8139cp driver has a change_mtu function that has not been
enabled since the dawn of the git repository. However, the
generic eth_change_mtu is not used in its place, so that
invalid MTU values can be set on the interface.

Original patch salvages the broken code for the single case of
setting the MTU while the interface is down, which is safe
and also includes the range check.  Now enhanced to support up
or down interface.

v2: fix case where rxbufsz isn't changed in the up state case

Original patch from
http://lkml.indiana.edu/hypermail/linux/kernel/1202.2/00770.html



Testing: has been test on virtual 8139cp setup without issue,
have no access real hardware 8139cp, need testing help.

Signed-off-by: "John Greene" <jogreene@redhat.com>
CC: "David S. Miller" <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>