ARM: davinci: uncompress.h: bail out if uart not initialized

Bail out of UART access functions in uncompress.h if the uart
port is not setup. This will happen when booting from DT since
machine type matching does not work in this case. This may also
happen if a correct machine type is not setup by bootloader.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>