b43: fix a lower bounds test

The problem is that "channel" is an unsigned int, when it's less 5 the
value of "channel - 5" is not a negative number as one would expect but
is very high positive value instead.

This means that "start" becomes a very high positive value.  The result
of that is that we never enter the "for (i = start; i <= end; i++) {"
loop.  Instead of storing the result from b43legacy_radio_aci_detect()
it just uses zero.

Fixes: ef1a628d ("b43: Implement dynamic PHY API")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Michael Büsch <m@bues.ch>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211006073621.GE8404@kili