serial: fsl_lpuart: fix maximum acceptable baud rate with over-sampling

Using over-sampling ratio, lpuart can accept baud rate upto uartclk / 4.

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