serial: fsl_lpuart: consider TX FIFO too in lpuart32_tx_empty

The commit 3876a00fcb6b ("tty: serial: fsl_lpuart: consider TX FIFO
too in tx_empty") fixed lpuart_tx_empty only.
Fix lpuart32_tx_empty too.

Signed-off-by: Atsushi Nemoto <atsushi.nemoto@sord.co.jp>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>