mac80211: minstrel_ht: sample_table can be __read_mostly

The sample table is initialized only once at module start, so
is really __read_mostly. Additionally, the code to init it can
be marked __init since it will never be needed again, it is
likely automatically inlined into the __init function already
by the compiler, so this doesn't really make a difference.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>