tty: rpmsg: Define tty name via constant string literal

Driver uses already twice the same string literal.

Define it in one place, so every user will have this
name consistent.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211025135148.53944-5-andriy.shevchenko@linux.intel.com


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