crypto: qat - remove unneeded assignment

The function adf_gen4_get_vf2pf_sources() computes a mask which is
stored in a variable which is returned and not used.
Remove superfluous assignment of variable.

This is to fix the following warning when compiling the QAT driver
with clang scan-build:

    drivers/crypto/qat/qat_common/adf_gen4_pfvf.c:46:9: warning: Although the value stored to 'sou' is used in the enclosing expression, the value is never actually read from 'sou' [deadcode.DeadStores]
            return sou &= ~mask;
                   ^      ~~~~~

Fixes: 5901b4af6e07 ("crypto: qat - fix access to PFVF interrupt registers for GEN4")
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>