USB: serial: clean up attribute permissions

Clean up the DEVICE_ATTR usage in the USB serial drivers, making them
more obvious as to the permissions that the sysfs files should be.

Note: ftdi_sio.c still has a DEVICE_ATTR() used, that will have to wait
until after 3.12-rc1 comes out when DEVICE_ATTR_WO() shows up in Linus's
tree.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>