sdcard: direct I/O file access fix

If a file is opened in direct I/O mode (with O_DIRECT flag),
the read buffer addess must be aligned to memory page size
boundary. The Direct I/O is not needed for normal files,
however, some special hardware access (e.g. smart SD cards)
will not work without it.

Change-Id: I42babeee86dba1880fd23e2592fddd7060da3e20