Revert "drivers: serial: uart_mcux_lpuart: fix IRQ-driven API with ASYNC_API"

This reverts commit a80fa516.

That commit broke the polling API when used with the interrupt driven,
it is completely wrong to disable the receiver from irq_rx_disable,
the uart_basic_api test has been broken on all lpuart platforms because
of this bug.

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