Bluetooth: hci_ldisc: Use speed set by btattach as oper_speed

Starting a BCM UART controller not defined as a platform device or
a serdev with "btattach -B /dev/ttyS1 -P bcm -S 3000000" works fine
but the serial port remains at the init_speed, i.e. 115200.

The oper_speed is only set if a device is declared in ACPI, device
tree or as a platform device.

This commit copies the serial port speed fixed by hciattach to the
oper_speed on line discipline opening.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>