ipw2x00: remove set but not used variable 'force_update'

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

drivers/net/wireless/intel/ipw2x00/ipw2100.c: In function shim__set_security:
drivers/net/wireless/intel/ipw2x00/ipw2100.c:5582:9: warning: variable force_update set but not used [-Wunused-but-set-variable]

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>