hostap: Remove unused variable 'fc'

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

 In file included from drivers/net/wireless/intersil/hostap/hostap_cs.c:196:
 drivers/net/wireless/intersil/hostap/hostap_hw.c: In function ‘prism2_tx_80211’:
 drivers/net/wireless/intersil/hostap/hostap_hw.c:1806:18: warning: variable ‘fc’ set but not used [-Wunused-but-set-variable]
 1806 | u16 tx_control, fc;
 | ^~
 In file included from drivers/net/wireless/intersil/hostap/hostap_plx.c:264:
 drivers/net/wireless/intersil/hostap/hostap_hw.c: In function ‘prism2_tx_80211’:
 drivers/net/wireless/intersil/hostap/hostap_hw.c:1806:18: warning: variable ‘fc’ set but not used [-Wunused-but-set-variable]
 1806 | u16 tx_control, fc;
 | ^~
 In file included from drivers/net/wireless/intersil/hostap/hostap_pci.c:221:
 drivers/net/wireless/intersil/hostap/hostap_hw.c: In function ‘prism2_tx_80211’:
 drivers/net/wireless/intersil/hostap/hostap_hw.c:1806:18: warning: variable ‘fc’ set but not used [-Wunused-but-set-variable]
 1806 | u16 tx_control, fc;
 | ^~

Cc: Jouni Malinen <j@w1.fi>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Taehee Yoo <ap420073@gmail.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/20200826093401.1458456-20-lee.jones@linaro.org