USB: serial: replace symbolic permissions by octal permissions

Replace symbolic permission macros with octal permission numbers
because octal permission numbers are easier to read and understand
instead of their symbolic macro names.

No functional change.

Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Utkarsh Verma <utkarshverma294@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>