osst: update ppos instead of using file->f_pos

osst_read()/osst_write() modify file->f_pos directly instead of the ppos
given to them.  The VFS later updates the file->f_pos and overwrites it
with the value of ppos.

Signed-off-by: Jan Blunck <jblunck@suse.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Willem Riede <osst@riede.org>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>