perf/x86/uncore: Fix compilation warning in snb_uncore_imc_init_box()

This patch fixes a compilation problem (unused variable) with the
new SNB/IVB/HSW uncore IMC code.

[ In -v2 we simplify the fix as suggested by Peter Zjilstra. ]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140311235329.GA28624@quad
Signed-off-by: Ingo Molnar <mingo@kernel.org>