omap: Add back UART MDR1 check into uncompress.h

Recent DEBUG_LL and uncompress.h changes removed the check_port()
as pointed out by Cory Maccarrone <darkstar6262@gmail.com>.

This causes some boards to not boot, so add back the MDR1 register
check. The MDR1 register tells the mode of omap uart. Based on
an earlier patch by Cory Maccarrone <darkstar6262@gmail.com>.

Tested-by: Cory Maccarrone <darkstar6262@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>