rtlwifi: btcoex: use %*ph to print small buffer

Use %*ph format to print small buffer as hex string.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200730154026.39901-1-andriy.shevchenko@linux.intel.com