usb: musb: dsps: fix res_name length

The res_name is used for the name construction of a DT property as
follows:

  sprintf(res_name, "port%d-mode", id);

Hence, res_name must be at least 11 characters long in order to store
the name including the terminating '\0'.

While at it, use to snprintf() rather than sprintf() when accessing this
buffer.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>