net80211: fix build for 526370fb85db4b659cff4625eb2f379acaa4a1a8

In 526370fb85db4b659cff4625eb2f379acaa4a1a8 "net80211: proper ssid
length check in setmlme_assoc_adhoc()" we are checking the
sizeof on an array function parameter which leads to a warning that
it will resturn the size of the type of the array rather than the
array size itself. Use the defined length used both in the ioctl
and the sizing of the array function parameter instead.

Reported by: CI
MFC after: 3 days
X-MFC with: 526370fb85db4b659cff4625eb2f379acaa4a1a8