When a platform is running at high frequencies it's not always possible to
scale-down a frequency to a requested value, and using mmc_spi driver this
leads to the following printk flood during card polling:

  ...
  mmc_spi spi32766.0: Requested speed is too low: 400000 Hz. Will use
  520828 Hz instead.
  mmc_spi spi32766.0: Requested speed is too low: 400000 Hz. Will use
  520828 Hz instead.
  ...

Fix this by using WARN_ONCE(), it's better than the flood, and also better
than turning dev_err() into dev_dbg(), since we actually want to warn that
some things may not work correctly.
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: Kumar Gala <galak@gate.crashing.org>
Cc: David Brownell <david-b@pacbell.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>