clk: replace pr_warn() with pr_err() for fatal cases

These three cases let clk_register() fail.  They should be considered
as error messages.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>