ethtool: fix application of verbose no_mask bitset

A bitset without mask in a _SET request means we want exactly the bits in
the bitset to be set. This works correctly for compact format but when
verbose format is parsed, ethnl_update_bitset32_verbose() only sets the
bits present in the request bitset but does not clear the rest. This can
cause incorrect results like

  lion:~ # ethtool eth0 | grep Wake
          Supports Wake-on: pumbg
          Wake-on: g
  lion:~ # ethtool -s eth0 wol u
  lion:~ # ethtool eth0 | grep Wake
          Supports Wake-on: pumbg
          Wake-on: ug

when the second ethtool command issues request

ETHTOOL_MSG_WOL_SET
    ETHTOOL_A_WOL_HEADER
        ETHTOOL_A_HEADER_DEV_NAME = "eth0"
    ETHTOOL_A_WOL_MODES
        ETHTOOL_A_BITSET_NOMASK
        ETHTOOL_A_BITSET_BITS
            ETHTOOL_A_BITSET_BITS_BIT
                ETHTOOL_BITSET_BIT_INDEX = 1

Fix the logic by clearing the whole target bitmap before we start iterating
through the request bits.

Fixes: 10b518d4

 ("ethtool: netlink bitset handling")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>