mmc: let mmc_of_parse() fail for insane bus-width value

You must fix your DT if it specifies insane bus-width, for example,
  bus-width = <3>;

debug() is not displayed in usual configuration, so people will not
even notice weirdness.  Use dev_err() instead, then let it fail.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>