[media] davinci: vpfe: fix return value check in vpfe_enable_clock()

[media] davinci: vpfe: fix return value check in vpfe_enable_clock()

In case of error, the function clk_get() returns ERR_PTR()
and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Reviewed-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>