ath9k: remove unused array firstep_table

Array firstep_table is being assigned but is never used
hence it is redundant and can be removed.

Cleans up clang warning:
warning: 'firstep_table' defined but not used [-Wunused-const-variable=]

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