ath5k: fix exp off-by-one when computing OFDM delta slope

Commit e8f055f0

 ("ath5k: Update reset code") subtly changed the
code that computes floating point values for the PHY3_TIMING register
such that the exponent is off by a decimal point, which can cause
problems with OFDM channel operation.

get_bitmask_order() actually returns the highest bit set plus one,
whereas the previous code wanted the highest bit set.  Instead, use
ilog2 which is what this code is really calculating.  Also check
coef_scaled to handle the (invalid) case where we need log2(0).

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>