n1sdp: set i2c slave address only when transaction is setup

This patch fixes the slave APIs such that the slave address will
be setup on every transaction and will be cleared when transaction
is complete.

This is done to avoid slave acknowledging the master when no
transaction was setup in slave side.

Change-Id: I53fb751891eb986480f91db636a7038fbe367c4f
Signed-off-by: Manoj Kumar <manoj.kumar3@arm.com>