MLK-10116 tty: serial: imx: fix flush buffer issue when module clock is at 4Mhz

In general, uart module clock require it is great than 80Mhz to match 5Mbps
baud rate. When test below 14Mhz module clock, software reset cause state
machines off normal. And for i.MX6SL evk board low power test, it set uart
module clock to 4Mhz, which cause console port print out messy code.
The patch just is workaround to fix console issue.

Signed-off-by: Fugang Duan <B38611@freescale.com>