coresight: trbe: Prevent overflow in PERF_IDX2OFF()

[ Upstream commit 105f5687 ]

Cast nr_pages to unsigned long to avoid overflow when handling large
AUX buffer sizes (>= 2 GiB).

Fixes: 3fbf7f01 ("coresight: sink: Add TRBE driver")
Signed-off-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>