staging: rtl8188eu: Remove function _rtw_writeN()

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

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