ibmvnic: simplify reset_long_term_buff function

The only thing reset_long_term_buff() should do is set
buffer to zero. After doing that, it is not necessary to
send_request_map again to VIOS since it actually does not
change the mapping. So, keep memset function and remove all
others.

Signed-off-by: Lijun Pan <ljp@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>