stmmac: review private structure fields

recently many new supports have been added in the stmmac driver w/o taking care
about where each new field had to be placed inside the private structure for
guaranteeing the best cache usage.
This is what I wanted in the beginning, so this patch reorganizes all the fields
in order to keep adjacent fields for cache effect.
I have also tried to optimize them by using pahole.

V2: do not abuse with ____cacheline_aligned_in_smp and keep fields that
potentially could stay in the same cache-line for better usage in SMP systems.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>