powerpc/mpc5200: Bugfix on handling variable sized buffer descriptors

The buffer descriptors for the ATA BestComm task are larger than the
current definition for bcom_bd.  This causes problems because the
various bcom_... functions dereference the buffer descriptor pointer
by using the array operator which doesn't work when the buffer
descriptors are a different size.

This patch adds the bcom_get_bd() function which uses the value in
bcom_task.bd_size to calculate the offset into the BD table.  This
patch also changes the definition of bcom_bd to specify a data size
of 0 instead of 1 so that it will never work if anyone attempts to
dereference the bd list as an array (as opposed to something that
might work even though it is wrong).

Finally, this patch moves the definition of bcom_bd up in the file
to eliminate a forward declaration.

Based on patch originally written by Tim Yamin.

Signed-off-by: Tim Yamin <plasm@roo.me.uk>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>