linux-yocto: Fix devupstream SRCREV assignment

Fixes the following error when building linux-yocto-upstream:

    Fetcher failure: Unable to find revision 570e7aedcd00e7e8a218e37c3dfb3b1fda83d111
    in branch v5.13/base even from upstream

The error occurred as the SRCREV_machine assignment earlier in the
recipe was being used instead of the mainline commit hash we want to use
for linux-yocto-upstream.

Signed-off-by: Paul Barker <paul@pbarker.dev>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>