rtlwifi: rtl8192c: phy_common: Remove unused variable 'bbvalue'

Fixes the following W=1 kernel build warning(s):

 drivers/net/wireless/realtek/rtlwifi/rtl8192c/phy_common.c: In function ‘_rtl92c_phy_iq_calibrate’:
 drivers/net/wireless/realtek/rtlwifi/rtl8192c/phy_common.c:1223:6: warning: variable ‘bbvalue’ set but not used [-Wunused-but-set-variable]

Cc: Ping-Ke Shih <pkshih@realtek.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: zhengbin <zhengbin13@huawei.com>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200821071644.109970-30-lee.jones@linaro.org