rockchip: misc: fix misc_read() return check

misc_read() is documented to return the number of bytes read or a
negative error value.  The Rockchip drivers currently do not implement
this correctly and instead return zero on success or a negative error
value.

In preparation for fixing the drivers, fix the condition here to only
error on negative values.

Suggested-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>