efi_loader: Use %pD to log device-path instead of local efi_dp_str()

Use %pD to log device-path instead of using efi_dp_str() and
efi_free_pool() locally in find_boot_device().
This is a cleanup patch, no feature update nor fix.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>