tests: littlefs: fix test_fs_mkfs_custom littlefs_init_cfg() fail

Fixes the littleFS test_fs_mkfs_custom test fail
in littlefs_init_cfg(): "erase size must be multiple of write size".
It happens for platforms with erase size < 2*write size (e.g. lpc55s).

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>