Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.c: In function brcmf_update_bss_info:
drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.c:2962:5: warning: variable dtim_period set but not used [-Wunused-but-set-variable]
drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.c: In function brcmf_update_bss_info:
drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.c:2961:6: warning: variable beacon_interval set but not used [-Wunused-but-set-variable]

They are never used so can be removed.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>