uboot-imx

Revert "Revert "stm32f4: fix serial output""

Due to a misunderstanding, in 698a12b we reverted changes that we in
fact wanted to keep.  So lets fix that mistake and bring the code back
to how it should have been.

This reverts commit 698a12bef9e782dcd99c555a739c16eec8669f14.

Signed-off-by: Tom Rini <trini@konsulko.com>