ddr: altera: Clean up find_vfifo_read()

ddr: altera: Clean up find_vfifo_read()

Fix data types, constify where applicable, fix comments and debug
strings. In case enough failing reads are detected, just return
right away instead of breaking from the loop. This lets us make
the error handling at the end much simpler.

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