dpll: Add an assertion to check freq_supported_num

dpll: Add an assertion to check freq_supported_num

Since the driver is broken in the case that src->freq_supported is not
NULL but src->freq_supported_num is 0, add an assertion for it.

Signed-off-by: Jiasheng Jiang <jiashengjiangcool@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Link: https://patch.msgid.link/20250228150210.34404-1-jiashengjiangcool@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>