Bluetooth: tester: Increase prepare write buffers pool

TP/GAW/SR/BV-10-C test case requires two long values to perform
nested prepare write operation. Using one ATT Execute Write command
this two values are going to be written simultaneously.

Change-Id: I2950409ed8f1d121dcc706b432cdf071b9af00c0
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>