thermal: bcm2835: Fix crash in bcm2835_thermal_debugfs

"cat /sys/kernel/debug/bcm2835_thermal/regset" causes a NULL pointer
dereference in bcm2835_thermal_debugfs. The driver makes use of the
implementation details of the thermal framework to retrieve a pointer
to its private data from a struct thermal_zone_device, and gets it
wrong - leading to the crash. Instead, store its private data as the
drvdata and retrieve the thermal_zone_device pointer from it.

Fixes: bcb7dd9e ("thermal: bcm2835: add thermal driver for bcm2835 SoC")

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>