Bluetooth: use configured params for ext adv

When the extended advertisement feature is enabled, a hardcoded min and
max interval of 0x8000 is used.  This patch fixes this issue by using
the configured min/max value.

This was validated by setting min/max in main.conf and making sure the
right setting is applied:

< HCI Command: LE Set Extended Advertising Parameters (0x08|0x0036) plen
25                                          #93 [hci0] 10.953011
…
Min advertising interval: 181.250 msec (0x0122)
Max advertising interval: 181.250 msec (0x0122)
…

Signed-off-by: Alain Michaud <alainm@chromium.org>
Reviewed-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Reviewed-by: Daniel Winkler <danielwinkler@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>