tests: bluetooth: tester: Fix GATT read multiple

Pass only speficied number of handler to read_params for GATT read
multiple. This is to avoid sending invalid (zeros) handles in
ATT_READ_MULTIPLE_REQ.

This was affecting GATT/CL/GAR/BI-18-C and GATT/CL/GAR/BI-19-C
qualification test cases.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>