tg3: Remove unnecessary spinlock

The spinlock is not needed after conversion of tg3_flags from array to
set_bit().

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