mfd: atmel-hlcdc: Do not sleep in atomic context

readl_poll_timeout() calls usleep_range(), but
regmap_atmel_hlcdc_reg_write() is called in atomic context (regmap
spinlock held).

Replace the readl_poll_timeout() call by readl_poll_timeout_atomic().

Cc: <stable@vger.kernel.org>
Fixes: ea31c0cf ("mfd: atmel-hlcdc: Implement config synchronization")
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>