tty: serial: samsung: Increase maximum baudrate

This driver can be used to communicate with Bluetooth chip in high-speed
UART mode, so increase the maximum baudrate to 3Mbps.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
[mszyprow: rephrased commit message]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>