staging: rtl8723bs: Remove function rtw_modular64()

Remove function rtw_modular64 as all it does is call do_div.
Modify call sites accordingly.
Issue found with Coccinelle.

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