i9e: Zero out private pointer on open.

i9e: Zero out private pointer on open.

This should not matter at the moment since i9e_open() is only called
once and the i9e_private structure is static, so it's already zeroed
out by the first time the function is called. But since the ie9 API
is supposed to work across multiple open/close cycles, it seems wise
to be conservative here.