samples: bluetooth: peripheral: Fix RX buffer size

GATT long messages require buffers to store multiple packets. The actual
configuration for the bluetooth peripheral sample defines only two
buffers to store these messages. As the default ATT payload size is
18 bytes, the maximum payload size for a long ATT message is 36 bytes.
As the variable `vnd_long_value[]` has a defined size of 74 bytes, it
cannot be fully modified due to the 36 bytes limitation.

Set CONFIG_BT_ATT_PREPARE_COUNT=5 so all the maximum payload size goes
to 90 bytes and all the 74 bytes can be written to `vnd_long_value[]`.

Signed-off-by: Joao Cordeiro <jvcc@cesar.org.br>