media: rockchip: rga: Fix possible ERR_PTR dereference in rga_buf_init()

In the Linux kernel, the following vulnerability has been resolved: media: rockchip: rga: Fix possible ERR_PTR dereference in rga_buf_init() rga_get_frame() can return ERR_PTR(-EINVAL) when buffer type is unsupported or invalid. rga_buf_init() does not check the return value and unconditionally dereferences the pointer when accessing f->size. Add proper ERR_PTR checking and return the error to prevent dereferencing an invalid pointer.