clk: ti: dflt: fix enable_reg validity check

The default clock enabling functions for TI clocks -
omap2_dflt_clk_enable() and omap2_dflt_clk_disable() perform a
NULL check for the enable_reg field of the clk_hw_omap structure.
This enable_reg field however is merely a combination of the index
of the master IP module, and the offset from the master IP module's
base address. A value of 0 is perfectly valid, and the current error
checking will fail in these cases. The issue was found when trying
to enable the iva2_ck clock on OMAP3 platforms.

So, switch the check to use IS_ERR. This correction is similar to the
logic used in commit c807dbed ("clk: ti: fix ti_clk_get_reg_addr
error handling").

Fixes: 9f37e90e ("clk: ti: dflt: move support for default gate clock..")
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>