bluetooth: tester: Increase SDU size to meet PTS requirement

PTS test cases MESH/NODE/TNPT/BV-07-C, MESH/NODE/TNPT/BV-12-C, requires
to receive 8 segment of data which summary requires 108 bytes of SDU.
This patch make test cases PASSing.

Signed-off-by: Grzegorz Kolodziejczyk <grzegorz.kolodziejczyk@codecoup.pl>