wifi: bypass attribute lookup for max Ampdu and Amsdu sizes

Splitting this from MR !681 (merged) as it isn't directly related. Along with MR 681, should provide a 1.24x speedup on optimized test.py times.

test.py times

real    2m49.718s
user    33m37.859s
sys     0m58.930s

real    2m52.075s
user    33m34.395s
sys     1m1.929s

real    2m42.315s
user    33m36.812s
sys     0m58.762s

real    2m45.858s
user    33m33.441s
sys     1m0.059s