alim15x3: fix PIO mode setup

The driver's tuneproc() method fails to set the drive's own speed -- fix this
by renaming the function to ali15x3_tune_pio() and "wrapping" the new tuneproc()
method around it and making it return the mode set, update the heading comment.

Also, setting PIO mode via the speedproc() method does not work due to passing 
to the tuneproc() method's a mode number not biased by XFER_PIO_0 -- fix this
along with a typo in the heading comment...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>