crypto: ccree - split overloaded usage of irq field

We were using the irq field of the drvdata struct in
an overloaded fahsion - saving the IRQ number during init
and then storing the pending itnerrupt sources during
interrupt in the same field.

This worked because these usage are mutually exclusive but
are confusing. So simplify the code and change the init use
case to use a simple local variable.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>