Bluetooth: shell: Fix illegal access

BT_LE_ADV_CONN_DIR_LOW_DUTY declares a local variable so it cannot go
out of scope, so instead of assigning in the scope of the if statement
this rework the code to mantain the declaration in the same scope it is
used.

Fixes #10570

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>