staging: rtl8723bs: Remove set but not used variable 'prwskeylen'

staging: rtl8723bs: Remove set but not used variable 'prwskeylen'

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

drivers/staging/rtl8723bs/core/rtw_security.c: In function rtw_tkip_encrypt:
drivers/staging/rtl8723bs/core/rtw_security.c:660:6: warning: variable prwskeylen set but not used [-Wunused-but-set-variable]
drivers/staging/rtl8723bs/core/rtw_security.c: In function rtw_tkip_decrypt:
drivers/staging/rtl8723bs/core/rtw_security.c:768:6: warning: variable prwskeylen set but not used [-Wunused-but-set-variable]
drivers/staging/rtl8723bs/core/rtw_security.c: In function rtw_aes_encrypt:
drivers/staging/rtl8723bs/core/rtw_security.c:1528:6: warning: variable prwskeylen set but not used [-Wunused-but-set-variable]

It is not used since commit 554c0a3abf21 ("staging:
Add rtl8723bs sdio wifi driver")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Link: https://lore.kernel.org/r/1570352999-45790-4-git-send-email-zhengbin13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>