staging: rtl8188eu: Remove function _rtw_read_port_cancel()

_rtw_read_port_cancel() is a wrapper function, being used to call
usb_read_port_cancel().
Call usb_read_port_cancel() directly and drop _rtw_read_port_cancel().

Signed-off-by: navin patidar <navin.patidar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>