fs: littlefs: Fix cache and lookahead size checks

This fixes an issue where wrong values were checked against block device
defaults. Kconfig values are used when no filesystem config values are
provided, and the buffers are sized according to the Kconfig values,
but the checks were only performed against filesystem config variables.

Signed-off-by: Djordje Nedic <nedic.djordje2@gmail.com>