staging: r8188eu: remove mutex 'usb_vendor_req_mutex'

staging: r8188eu: remove mutex 'usb_vendor_req_mutex'

Remove mutex 'usb_vendor_req_mutex'.

It was used to protect a shared buffer for USB requests and, since that
buffer is removed in previous patch, this mutex is now useless.

Furthermore, because it was used to serialize the calls to the Core USB
API, we thoroughly tested the enabling of concurrent firing of USB requests
without the mutex and found no problems of any kind in common use cases.

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/20210924122705.3781-17-fmdefrancesco@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>