drivers: uart_mcux: Do not select async support

Commit f1b0b458 mistakenly selected the
SERIAL_SUPPORT_ASYNC kconfig for the kinetis uart
driver when enabling the lpuart driver. Revert this,
because the kinetis uart driver does not support async api.

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