Staging: rtl8723au: Use USB API functions

This patch replaces driver specific functions(RT_usb_endpoint_is_bulk_in,
RT_usb_endpoint_is_bulk_out, RT_usb_endpoint_is_int_in) with USB API
functions(usb_endpoint_is_bulk_in, usb_endpoint_is_bulk_out,
usb_endpoint_is_int_in).

Also, this patch removes these three RT functions as they are no longer
needed after replacement.

This patch also have one line over 80 characters as limiting it to 80
characters does not make code look good.

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>