crypto: caam/qi - fix warning in init_cgr()

Coverity warns about an
"Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)"
when computing the congestion threshold value.

Even though it is highly unlikely for an overflow to happen,
use this as an opportunity to simplify the code.

Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>