clk: versatile: Remove WARNs in ->round_rate()

clk_round_rate() is intended to be used to round a given clock rate to
the closest one achievable by the actual clock. This implies that the
input to clk_round_rate() is expected to be unachievable - and such
cases shouldn't be treated as exceptional.

To reflect this, remove the WARN_ONs which trigger when an unachievable
clock rate is passed to vexpress_osc_round_rate().

Reported-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>