bootm: Disable interrupts before loading OS

This restores the ordering of interrupt disable to what it what before
commit 35fc84fa. It seems that on some archiectures (e.g. PowerPC) the
OS is loaded into an interrupt region, which can cause problems if
interrupts are still running.

Tested-by: Stefan Roese <sr@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>