In case errno contains some non-zero value and call to strtoul() is
successful -2 error will be returned.
Setting errno to zero before calling strtoul() makes sure
errors will be handled correctly.
Fixes: 0ddc9b815b11 ("net/mrvl: add net PMD skeleton")
Signed-off-by: Tomasz Duszynski <tdu@semihalf.com>