serial: 8250_mid: Remove unneeded test for ->setup() presence

All supported platforms by this driver require ->setup() and ->exit().
Remove unneeded test for ->setup() presence.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220215101111.47250-1-andriy.shevchenko@linux.intel.com


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