hw/misc/bcm2835_cprman: correct register index in test for mux update

After each register write handled by cprman_write, function update_mux
checks each of the clocks to see whether the supplied register index
applies to it, calling clock_mux_update if so. Each clock has two
registers, so it needs to check the base cm_offset for CM_CTL and then
the next slot for CM_DIV. But, it was checking for cm_offset plus 4, as
though it were a byte offset when it's a logical index.

Signed-off-by: Jonathan Gilbert <logic@deltaq.org>
Fixes: 7281362484a ("hw/misc/bcm2835_cprman: clock mux skeleton  implementation")
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Message-ID: <20260925100726.1546566-1-logic@deltaq.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>