powerpc/powermac: Remove variable x that's never read

Since the value of x is never intended to be read, remove it. Fix
warning treated as error with W=1:

  arch/powerpc/platforms/powermac/udbg_scc.c:76:9: error: variable ‘x’ set but not used

Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>