staging: rtl8723bs: hal: Replace function rtl8723b_set_rssi_cmd()

Remove function rtl8723b_set_rssi_cmd as all it does is call
rtl8723b_set_FwRssiSetting_cmd.
Rename rtl8723b_set_FwRssiSetting_cmd to rtl8723b_set_rssi_cmd and
change its type from static to non-static to maintain compatibility
with call sites.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>