Fix build errors when CONFIG_LOGBUFFER and CONFIG_FIT are enabled

Recent modifcations to LOGBUFFER handling code were incorrecly
introduced to fit_check_kernel() routine during
"Merge branch 'new-image' of git://www.denx.de/git/u-boot-testing

",
commit 27f33e9f.

This patch cleans up this merge issue.

Signed-off-by: Marian Balakowicz <m8@semihalf.com>