tc-testing: skbmod: fix match value of ethertype

iproute2 print_skbmod() prints the configured ethertype using format 0x%X:
therefore, test 9aa8 systematically fails, because it configures action #4
using ethertype 0x0031, and expects 0x0031 when it reads it back. Changing
the expected value to 0x31 lets the test result 'not ok' become 'ok'.

tested with:
 # ./tdc.py -e 9aa8
 Test 9aa8: Get a single skbmod action from a list
 All test results:

 1..1
 ok 1 9aa8 Get a single skbmod action from a list

Fixes: cf797ac4

 ("tc-testing: Add test cases for police and skbmod")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>