tg3: Fix deadlock in tg3_change_mtu()

tg3: Fix deadlock in tg3_change_mtu()

Quoting David Vrabel -
"5780 cards cannot have jumbo frames and TSO enabled together.  When
jumbo frames are enabled by setting the MTU, the TSO feature must be
cleared.  This is done indirectly by calling netdev_update_features()
which will call tg3_fix_features() to actually clear the flags.

netdev_update_features() will also trigger a new netlink message for the
feature change event which will result in a call to tg3_get_stats64()
which deadlocks on the tg3 lock."

tg3_set_mtu() does not need to be under the tg3 lock since converting
the flags to use set_bit(). Move it out to after tg3_netif_stop().

Reported-by: David Vrabel <david.vrabel@citrix.com>
Tested-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>