carl9170: clean up a clamp() call

The parameter order for clamp is supposed to be clamp(value, low, high).
When we write it in this order it's equivalent to
min(head->plcp[3] & 0x7f, 75) which works in this context where the min
is zero.  But it's not a correct use of the API.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>