OMAP: hwmod: Remove IS_ERR check with omap_clk_get_by_name return value

The previous clock API was returning a standard linux error code in
case of failure. This is not the case anymore with the new
omap_clk_get_by_name API. A NULL value means that the clock node
does not exist.
Replace all the IS_ERR check by a !clk check.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>