clk: Replace explicit clk assignment with __clk_hw_set_clk

The change in the clk API to return a per-user clock instance, moved
the clock state to struct clk_core so now the struct clk_hw .core field
is used instead of .clk for most operations.

So for hardware clocks that needs to share the same clock state, both
the .core and .clk pointers have to be assigned but currently only the
.clk is set. This leads to NULL pointer dereference when the operations
try to access the hw clock .core. For example, the composite clock rate
and mux components didn't have a .core set which leads to this error:

Unable to handle kernel NULL pointer dereference at virtual address 00000034
pgd = c0004000
[00000034] *pgd=00000000
Internal error: Oops: 5 [#1] PREEMPT SMP ARM
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.19.0-next-20150211-00002-g1fb7f0e1150d #423
Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
task: ee480000 ti: ee488000 task.ti: ee488000
PC is at clk_mux_determine_rate_flags+0x14/0x19c
...