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>

drivers/net/wireless/ath/ath9k/ar5008_phy.c

@@ -37,10 +37,6 @@
 #define AR5008_11NG_HT_SS_SHIFT		12
 #define AR5008_11NG_HT_DS_SHIFT		20
 
-static const int firstep_table[] =
-/* level:  0   1   2   3   4   5   6   7   8  */
-	{ -4, -2,  0,  2,  4,  6,  8, 10, 12 }; /* lvl 0-8, default 2 */
-
 /*
  * register values to turn OFDM weak signal detection OFF
  */