Bluetooth: Add variable SSP auto-accept delay support

Some test systems require an arbitrary delay to the auto-accept test
cases for Secure Simple Pairing in order for the tests to pass.
Previously when this was handled in user space it was worked around by
code modifications and recompilation, but now that it's on the kernel
side it's more convenient if there's a debugfs interface for it.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>