ipmid: Fix Get BT Interface Capabilities response
ipmid: Fix Get BT Interface Capabilities response

Per the IPMI 2.0 spec, the input and output buffer size
must be maximum buffer size minus one byte to allocate
space for the length byte.

Change-Id: Ie1768b9c6844275f7897fbf8056ce2a19b1c11c3
Fixes: openbmc/openbmc#847
Signed-off-by: Adriana Kobylak <742cf4d8dead98b4cab50b088fd2a1406293c958@us.ibm.com>

openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid