ath5k: remove redundant pointer rf

Pointer rf is being assigned but is never used hence it is redundant
and can be removed.

Cleans up two clang warnings:
warning: variable 'rf' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>