Don't bypass cache in lfs_cache_prog() and lfs_cache_read()

In some cases specific alignment of buffer passed to underlying device
is required. For example SDMMC in STM32F7 (when used with DMA) requires
the buffers to be aligned to 16 bytes. If you enable data cache in
STM32F7, the alignment of buffer passed to any driver which uses DMA
should generally be at least 32 bytes.

While it is possible to provide sufficiently aligned "read", "prog" and
per-file caches to littlefs, the cases where caches are bypassed are
hard to control when littlefs is hidden under some additional layers.
For example if you couple littlefs with stdio and use it via `FILE*`,
then littlefs functions will operate on internal `FIlE*` buffer, usually
allocated dynamically, so in these specific cases - with insufficient
alignment (8 bytes on ARM Cortex-M).

The easy path was taken - remove all cases of cache bypassing.

Fixes #158

lfs.c

@@ -80,21 +80,6 @@ static int lfs_bd_read(lfs_t *lfs,
             diff = lfs_min(diff, rcache->off-off);
         }
 
-        if (size >= hint && off % lfs->cfg->read_size == 0 &&
-                size >= lfs->cfg->read_size) {
-            // bypass cache?
-            diff = lfs_aligndown(diff, lfs->cfg->read_size);
-            int err = lfs->cfg->read(lfs->cfg, block, off, data, diff);
-            if (err) {
-                return err;
-            }
-
-            data += diff;
-            off += diff;
-            size -= diff;
-            continue;
-        }
-
         // load to cache, first condition can no longer fail
         LFS_ASSERT(block < lfs->cfg->block_count);
         rcache->block = block;