coresight: Fix the coresight_enable_reg_clk() stub

coresight_enable_reg_clk() is supposed to return a value. Fix up the stub
to return -EINVAL.

Fixes: 0e89c08f ("coresight: Replace functions for tpdm's clocks/regulators")
Change-Id: Ic0dedbade05d4786e3f3d67fd7044785326f9b49
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>