boot: simplify bootmeth_vbe_simple_ft_fixup()

Don't assign a value to a variable if it is not used afterwards.
Move variables to the code fragment where they are used.

Addresses-Coverity: CID 356243 ("Code maintainability issues (UNUSED_VALUE)")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>