rockchip: otp: fix misc_read() return values

The documentation for misc_read() says:

    Return: number of bytes read if OK (may be 0 if EOF), -ve on error

The Rockchip efuse driver implements this so it should return the number
of bytes read rather than zero on success.  Fix this so that the driver
follows the usual contract for read operations.

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