cmd: clk: Check return value from soc_clk_dump

In case of error in soc_clk_dump function are returned different values
then CMD return values (-1, 0, 1).

For example:
ZynqMP> clk dump
exit not allowed from main input shel

The patch is checking all negative return values and return
CMD_RET_FAILURE which is proper reaction for these cases.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>