iw: 'phy <name> reg get' shouldn't dump all domains

iw: 'phy <name> reg get' shouldn't dump all domains

Right now, 'iw phy ... reg get' gets translated to NLM_F_DUMP, which
dumps all domains. But this command really is looking for non-DUMP
semantics -- it only wants the domain for the specified wiphy, or (per
the kernel's choice) the global, if the wiphy doesn't have its own.

We can achieve this by skipping the handle_reg_get() -> "reg dump"
indirection.

[ with a wiphy-specific domain ]
Before:
  # iw phy phy0 reg get
  global
  country XX: DFS-YYY
  [...]
  phy#0
  country AA: DFS-BBB
  [...]

After:
  # iw phy phy0 reg get
  phy#0
  country AA: DFS-BBB
  [...]

[ without a wiphy-specific domain ]
Before and after (unchanged):
  # iw phy phy0 reg get
  global
  country XX: DFS-YYY
  [...]

Signed-off-by: Brian Norris <briannorris@chromium.org>
Link: https://lore.kernel.org/r/20200331232833.142026-1-briannorris@chromium.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>