examples: fix port mask parsing failure handling

examples: fix port mask parsing failure handling

Giving invalid or zero portmask as command line option to
these applications will have an unexpected response.
The reason behind this is that the return value of function
that parses portmask is stored in a variable whose datatype is
unsigned int, hence returning -1 in case of zero or
invalid portmask causes an unexpected behaviour.
If we return 0 instead of -1 this issue can be resolved.
The program already contains the functionality to print
"invalid portmask" and program usage if zero is returned.

Signed-off-by: Sarosh Arif <sarosh.arif@emumba.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>