wifi: ray_cs: Utilize strnlen() in parse_addr()

Instead of doing simple operations and using an additional variable on stack,
utilize strnlen() and reuse len variable.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220603164414.48436-1-andriy.shevchenko@linux.intel.com