config: De-nest the code in Port.splice().

config: De-nest the code in Port.splice().

The error checking in that function used an if/else structure where
one of the two branches would be the error condition which would cause
the function to exit. Because the function would exit if an error was
detected, there's no reason to have the non-error condition guarded in
the other half of the if.

This change de-nests the non-error cases to make the function simpler
and easier to read.

Change-Id: Idedf54e84a178fa5a2a47f96373374152e420cf3
Reviewed-on: https://gem5-review.googlesource.com/c/15516
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>