mpc52xx: cleanup locking

commit 1e4e0767 (Fix locking on fec_mpc52xx driver) assumed IRQ are
enabled when an IRQ handler is called.

It is not the case anymore (IRQF_DISABLED is deprecated), so we can use
regular spin_lock(), no need for spin_lock_irqsave().

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Jean-Michel Hautbois <jhautbois@gmail.com>
Cc: Asier Llano <a.llano@ziv.es>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>