vbe: Convert some checks to assertions

vbe: Convert some checks to assertions

VBE is currently quite careful with function arguments because it is
used in VPL which cannot be updated after manufacture. Bugs can cause
security holes.

Unfortunately this adds to code size.

In several cases we are reading values from a devicetree which is part
of U-Boot (or at least VPL) and so known to be good. Also, in several
places, getting bad values does not matter.

So change a few checks to assert() to reduce code size.

Signed-off-by: Simon Glass <sjg@chromium.org>