ptp: ocp: Fix uninitialized variable warning spotted by clang.

If attempting to flash the firmware with a blob of size 0,
the entire write loop is skipped and the uninitialized err
is returned.  Fix by setting to 0 first.

Fixes: 773bda96

 ("ptp: ocp: Expose various resources on the timecard.")
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>