mtd: nand: r852: correct write_buf loop bounds

The two loops in r852_write_buf() are designed to handle 4-byte-aligned
and then 1-byte-aligned portions, respectively. However, there are two
issues:

(1) The first loop will only terminate if 'len' is a multiple of 4
(2) The second loop will never terminate if it runs at least once

Rewrite these loops as they were probably intended. Compile tested only.

Issues pointed out by Coverity Scan.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>