Allow framework to compile with BTM_IO_CAP_NONE

Using BTM_IO_CAP_NONE currently causes compilation to fail since
bta_dm_sp_cback ends up unused out a previous #ifdef that only kicks
in if BTM_IO_CAP_NONE is not set. Plugging the entire function into
a #ifdef so consistency is maintained and the framework can compile.

Bug: 35257446
Test: Built locally and run on device. Pairing without display using
JustWorks works.

Change-Id: Ibd7af6454253ccd731ff79602f327a52b8fe585b