.pytool/EccCheck: Check ecc_csv exists

.pytool/EccCheck: Check ecc_csv exists

'workspace_path' being an absolute path leads to 'ecc_csv' being
an absolute path. Then it won't be found among 'file' as they
are relative paths.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>