sky2: avoid pci write posting after disabling irqs

In sky2_change_mtu setting B0_IMSK to 0 may be delayed due to PCI write posting
which could result in irqs being still active when synchronize_irq is called.
Since we are not prepared to handle any further irqs after synchronize_irq
(our resources are freed after that) force the write by a consecutive read from
the same register.
Similar situation in sky2_all_down: Here we disabled irqs by a write to B0_IMSK
but did not ensure that this write took place before synchronize_irq. Fix that
too.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>