While polling for BOOT_DONE the chip could NAK a read because it is
still booting, which would terminate the regmap_read_poll_timeout()
with an error.

Instead implement a polling loop that ignores read errors so we
always poll until the chip signals boot or the loop times out.
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>