ipa: Fix debugfs tethering stats print

Currently the ipa_debugfs_print_tethering_stats()
prints values from zeroed buffer.

The ipa_get_teth_stats() has to be called only once,
and ipa_query_teth_stats() to fill the buffer.

Change-Id: I72e6db3c9f2c342e8b635341eee06795b2be9d67
Signed-off-by: Ilia Lin <ilialin@codeaurora.org>