mac80211_hwsim: don't shadow a global variable

The argument to hwsim_init_s1g_channels() shadows a global,
change that to be clearer.

Link: https://lore.kernel.org/r/20220214173004.66bac90d64a8.I3e878e42bf2feecbb0a6ca38a68c236c23a8c9e6@changeid


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