powerpc/booke: clear DBCR0_BT in user_disable_single_step()

BookE version of user_disable_single_step() clears DBCR0_IC for the
instruction completion debug, but did not also clear DBCR0_BT for the
branch taken exception.  This behavior was lost by the 2/2010 patch.

Signed-off-by: James Yang <James.Yang@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>