stmmac: Discard masked flags in interrupt status register

DW GMAC databook says the following about bits in "Register 15 (Interrupt
Mask Register)":
--------------------------->8-------------------------
When set, this bit __disables_the_assertion_of_the_interrupt_signal__
because of the setting of XXX bit in Register 14 (Interrupt
Status Register).
--------------------------->8-------------------------

In fact even if we mask one bit in the mask register it doesn't prevent
corresponding bit to appear in the status register, it only disables
interrupt generation for corresponding event.

But currently we expect a bit different behavior: status bits to be in
sync with their masks, i.e. if mask for bit A is set in the mask
register then bit A won't appear in the interrupt status register.

This was proven to be incorrect assumption, see discussion here [1].
That misunderstanding causes unexpected behaviour of the GMAC, for
example we were happy enough to just see bogus messages about link
state changes.

So from now on we'll be only checking bits that really may trigger an
interrupt.

[1] https://lkml.org/lkml/2016/11/3/413

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Fabrice Gasnier <fabrice.gasnier@st.com>
Cc: Joachim Eastwood <manabian@gmail.com>
Cc: Phil Reid <preid@electromag.com.au>
Cc: David Miller <davem@davemloft.net>
Cc: Alexandre Torgue <alexandre.torgue@gmail.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>