dpaa2-eth: prevent array underflow in update_cls_rule()

The "location" is controlled by the user via the ethtool_set_rxnfc()
function.  This update_cls_rule() function checks for array overflows
but it doesn't check if the value is negative.  I have changed the type
to unsigned to prevent array underflows.

Fixes: afb90dbb

 ("dpaa2-eth: Add ethtool support for flow classification")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>