staging: r8188eu: call new usb_write() from rtw_write{8,16,32,N}()

Create and call new usb_write() instead of usbctrl_vendorreq() from
inside rtw_write{8,16,32,N}().

In old code, rtw_write{8,16,32,N}() 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 is passed by callers.

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

A noteworthy feature of usb_control_msg_send() is that 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.

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

Suggested-by: Greg Kroah-Hartman <[email protected]>
Co-developed-by: Pavel Skripkin <[email protected]>
Signed-off-by: Pavel Skripkin <[email protected]>
Signed-off-by: Fabio M. De Francesco <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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