staging: r8188eu: remove the helpers of rtw_read16()

staging: r8188eu: remove the helpers of rtw_read16()

Remove the unnecessary _rtw_read16() and usb_read16() and embed their
code into the caller (i.e., rtw_read16()).

_rtw_read16() is a mere redefinition of rtw_read16() and it is unneeded.
usb_read16() was the only functions assigned to the (*_usb_read16) pointer,
so we can simply remove it and make a direct call.

This patch is in preparation for the _io_ops structure removal.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Co-developed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Link: https://lore.kernel.org/r/20210917071837.10926-4-fmdefrancesco@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>