ath5k: Fix range scaling when setting rate power table

rates[i] is unsigned but txp_offset can be negative for newer parts
with PDADC table. We cover the case when rates[i] + txp_offset > 63
but we must also cover the case when its < 0 or else rates[i] will overflow.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>