brcmsmac: ampdu: Remove a couple set but unused variables

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

 from drivers/net/wireless/broadcom/brcm80211/brcmsmac/ampdu.c:18:
 drivers/net/wireless/broadcom/brcm80211/brcmsmac/ampdu.c: In function ‘brcms_c_ampdu_dotxstatus_complete’:
 drivers/net/wireless/broadcom/brcm80211/brcmsmac/ampdu.c:850:7: warning: variable ‘update_rate’ set but not used [-Wunused-but-set-variable]
 drivers/net/wireless/broadcom/brcm80211/brcmsmac/ampdu.c: In function ‘brcms_c_ampdu_dotxstatus’:
 drivers/net/wireless/broadcom/brcm80211/brcmsmac/ampdu.c:1027:20: warning: variable ‘scb_ampdu’ set but not used [-Wunused-but-set-variable]

Cc: Arend van Spriel <arend.vanspriel@broadcom.com>
Cc: Franky Lin <franky.lin@broadcom.com>
Cc: Hante Meuleman <hante.meuleman@broadcom.com>
Cc: Chi-Hsien Lin <chi-hsien.lin@cypress.com>
Cc: Wright Feng <wright.feng@cypress.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: linux-wireless@vger.kernel.org
Cc: brcm80211-dev-list.pdl@broadcom.com
Cc: brcm80211-dev-list@cypress.com
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/20200826093401.1458456-3-lee.jones@linaro.org