staging: rtl8188eu: Remove function _rtw_write16()

_rtw_write16() is a wrapper function, being used to call usb_write16().
Call usb_write16() directly and drop _rtw_write16().

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