staging: r8188eu: call new usb_read() from rtw_read{8,16,32}()

staging: r8188eu: call new usb_read() from rtw_read{8,16,32}()

Create and call new usb_read() instead of usbctrl_vendorreq() from
inside rtw_read{8,16,32}().

In old code, rtw_read{8,16,32}() called usbctrl_vendorreq() which in
turn uses usb_control_msg() from within a "while" loop to build a control
URB, send it off and wait for completion. usbctrl_vendorreq() was used for
both receiving and sending messages, depending on the "requesttype"
argument which was passed by callers.

Compared to usbctrl_vendorreq(), which managed both reads and writes
from and to the USB endpoint, the new usb_read() manages only reads. For
this purpose it uses the newer USB Core usb_control_msg_recv() API. The
latter is preferred according both to a suggestion by Greg Kroah-Hartman
and also to its actual design.

Two noteworthy features of usb_control_msg_recv() are that (1) the data
pointer can be made to a reference on the stack because it does not have
the restriction that usb_control_msg() has where the data pointer must be
to dynamically allocated memory, and that (2) the whole message must be
properly received from the device in order for this function to be
successfuli (if a device returns less than the expected amount of data,
then the function will fail).

usbctrl_vendorreq() uses a "while" loop that we considered unnecessary
so that it is not in the new usb_read(). Furthermore, the latter has no
redundant checking, less obvious comments, and it manages errors before
success cases. All in all, usb_read() is simpler than
usbctrl_vendorreq() and uses less lines of code.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Co-developed-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20210924122705.3781-14-fmdefrancesco@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>