b43: N-PHY: increase initial value of "mind" in RSSI calibration

We're using "mind" variable to find the VCM that got the best polling
results. For each VCM we calculte "currd" which is compared to the
"mind". For PHY rev3+ "currd" gets values around 14k-40k. Looking for a
value smaller than 40 makes no sense, so increase the initial value.

This fixes a regression introduced in 3.4 by commit:
e0c9a021
(my BCM4322 performance dropped from 18,4Mb/s to 9,26Mb/s)

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>