ath5k/phy.c: fix negative array index

Author: Adrian Bunk <bunk@kernel.org>

This patch fixes a negative array index spotted by the Coverity checker.

Changes-licensed-under: ISC

Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@winlab.rutgers.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>