have_ca indicates whether we have an explicit CA cert; don't set it
when the CA is from a PKCS#12 file.
Without this fix, trying to import a profile with a PKCS#12 that includes the CA certificate results in error:
"--ca can not be PKCS#12 format"because we first set have_pkcs12=TRUE, have_ca=TRUE and then we check
that the CA is not in PKCS#12 format.
Fixes: 34a0c925 ("Handle the various pkcs12/cert/key/ca combinations.")
cc: @slowbro