Bluetooth: Fix RFCOMM RPN negotiation

According to the ETSI 3GPP TS 07.10 the default bit rate value for RFCOMM
is 9600 bit/s. Return this bit rate in case of RPN request and accept other
sane bit rates proposed by the sender in RPM command.

Signed-off-by: Yuri Kululin <ext-yuri.kululin@nokia.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>