rtlwifi: Remove set but not used variable 'rtstate'

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

drivers/net/wireless/realtek/rtlwifi/ps.c: In function rtl_ps_set_rf_state:
drivers/net/wireless/realtek/rtlwifi/ps.c:71:19: warning: variable rtstate set but not used [-Wunused-but-set-variable]

It is not used since commit f1d2b4d3 ("rtlwifi:
rtl818x: Move drivers into new realtek directory")

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