brcmsmac: remove three set but not used variables

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

drivers/net/wireless/broadcom/brcm80211/brcmsmac/main.c: In function 'brcms_c_set_gmode':
drivers/net/wireless/broadcom/brcm80211/brcmsmac/main.c:5257:7: warning: variable 'preamble_restrict' set but not used [-Wunused-but-set-variable]
drivers/net/wireless/broadcom/brcm80211/brcmsmac/main.c:5256:6: warning: variable 'preamble' set but not used [-Wunused-but-set-variable]
drivers/net/wireless/broadcom/brcm80211/brcmsmac/main.c:5251:7: warning: variable 'shortslot_restrict' 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>