crypto: atmel - fix checks of error code returned by devm_ioremap_resource()

The change fixes potential oops while accessing iomem on invalid
address, if devm_ioremap_resource() fails due to some reason.

The devm_ioremap_resource() function returns ERR_PTR() and never
returns NULL, which makes useless a following check for NULL.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Fixes: b0e8b341 ("crypto: atmel - use devm_xxx() managed function")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>