mac80211_hwsim: use DEFINE_IDA

This is preferred to opencoding an IDA_INIT.

Link: http://lkml.kernel.org/r/20180313132639.17387-2-willy@infradead.org


Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>