cmd: clk: replace clk_lookup by uclass_get_device_by_name

The function clk_lookup can be replaced by a direct call
to uclass_get_device_by_name for UCLASS_CLK.

This patch removes duplicated codes by the generic DM API and avoids
issue in clk_lookup because result of uclass_get_device wasn't tested;
when ret < 0, dev = NULL and dev->name is invalid, the next function
call strcmp(name, dev->name) causes a crash.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20220131172131.2.I7bc7762eff1e31ab7ff5b34c416ee03b8fe52200@changeid