ddr: altera: Clean up rw_mgr_mem_calibrate_vfifo_center() part 4

ddr: altera: Clean up rw_mgr_mem_calibrate_vfifo_center() part 4

First of all, don't break strings in the debug_cond() calls across
multiple lines. Next, zap a couple of really odd casts in the code.
Finally, make i into a signed variable, so it is possible to avoid
constructs of the form:
    for (i = FOO;; i--) { <code>; if (i == 0) break; }.

Signed-off-by: Marek Vasut <marex@denx.de>