ns16550: properly gate Exar PCIe UART cards support

ns16550: properly gate Exar PCIe UART cards support

Arm is about to get PCI passthrough support which means CONFIG_HAS_PCI
will be enabled, so this code will fail as Arm doesn't have ns16550
PCI support:

ns16550.c:313:5: error: implicit declaration of function 'enable_exar_enhanced_bits' [-Werror=implicit-function-declaration]
  313 |     enable_exar_enhanced_bits(uart);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~

Fix this by gating Exar PCIe UART cards support with the above in mind.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>