clk: ti: dra7-atl-clock: Fix possible ERR_PTR dereference

of_clk_get_from_provider() returns ERR_PTR on failure. The
dra7-atl-clock driver was not checking its return value and
immediately used it in __clk_get_hw().  __clk_get_hw()
dereferences supplied clock, if it is not NULL, so in that case
it would dereference an ERR_PTR.

Fixes: 9ac33b0ce81f ("CLK: TI: Driver for DRA7 ATL (Audio Tracking Logic)")
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>