mtd: nand: denali: use upper/lower_32_bits() macro for clean-up

I used (uint64_t) cast to avoid "right shift count >= width of type"
warning.  <linux/kernel.h> provides nice helpers to cater to it.
The code will be cleaner, and easier to understand.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>