crypto: atmel - Avoid flush_scheduled_work() usage

Flushing system-wide workqueues is dangerous and will be forbidden.
Replace system_wq with local atmel_wq.

If CONFIG_CRYPTO_DEV_ATMEL_{I2C,ECC,SHA204A}=y, the ordering in Makefile
guarantees that module_init() for atmel-i2c runs before module_init()
for atmel-ecc and atmel-sha204a runs.

Link: https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>