spl: nand: sunxi: rework status polling loop

spl: nand: sunxi: rework status polling loop

check_value_xxx() helpers are using a 1ms delay between each test, which
can be quite long for some operations (like a page read on an SLC NAND).
Since we don't have anything to do but to poll this register, reduce the
delay between each test to 1us.

While we're at it, rename the max_number_of_retries parameters and the
MAX_RETRIES macro into timeout_us and DEFAULT_TIMEOUT_US to reflect that
we're actually waiting a given amount of time and not only a number of
retries.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>