mtd: nand: denali: use more FIELD_PREP / FIELD_GET where appropriate

In several places in this driver, the register fields are retrieved
as follows:

  val = reg & FOO_MASK;

Then, modified as follows:

  reg &= ~FOO_MASK;
  reg |= val;

This code relies on its shift is 0, which we will never know until
we check the definition of FOO_MASK.  Use FIELD_PREP / FIELD_GET
where appropriate.

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