Exit jz4770_cgu_init() if the 'cgu' pointer we get is NULL, since the
pointer is passed as argument to functions later on.

Fixes: 7a01c190 ("clk: Add Ingenic jz4770 CGU driver")
Cc: stable@vger.kernel.org
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lkml.kernel.org/r/20200213161952.37460-1-paul@crapouillou.netSigned-off-by: Stephen Boyd <sboyd@kernel.org>