mmc: rename mmc_host_clk_{ungate|gate} to mmc_host_clk_{hold|release}

As per suggestion by Linus Walleij:

  > If you think the names of the functions are confusing then
  > you may rename them, say like this:
  >
  > mmc_host_clk_ungate() -> mmc_host_clk_hold()
  > mmc_host_clk_gate() -> mmc_host_clk_release()
  >
  > Which would make the usecases more clear

(This is CC'd to stable@ because the next two patches, which fix
observable races, depend on it.)

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: <stable@kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>