fs: fat: eliminate DIRENTSPERBLOCK() macro

The FAT filesystem implementation uses several marcros referring to a magic
variable name mydata which renders the code less readable. Eliminate one of
them which is only used for a debug() statement.

Use log_debug() instead of debug().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>