drivers: uart_mcux_lpuart: Fix APIs used together

Handle controller instances wanting to use different
API instances, need to change init flow so that they dont
interfere with each other, trick for now is returning
either the interrupt driven or async apis as not enabled
if the other has already been used on that controller.

Previously, there was an issue where enabling CONFIG_UART_ASYNC_API
would overwrite the init even if controller was meant to use
interrupt driven (as kconfig is global to affect all the controllers)

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>