subsystem: kernel_shell: Fix pointer cast to u32_t

The current code is assuming that the pointer to the thread structure is
32bit, casting it to u32_t before printing its address. This is wrong on
64bit architectures (CONFIG_64BIT) and the compiler complains.

Fix the problem by using '%p' to print the address.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>