Fix setting custom baud rates under Linux

This regression was caused by 7c737e0edcb93585856c65890ef34e5c5a28ee6b.
The initialization of the (current) serial port info was removed in that commit.

Tested on Linux x86_64 with Qt 5.1.1 and 4.8.6.

Task-number: QTBUG-38961
Change-Id: Idd3c5dc7eb37093848f33387563ff14567ac54fe
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Merge remote-tracking branch 'origin/5.3.0' into 5.3

Manually adjusted:
    src/serialport/qserialport_unix.cpp
    to remove isCustomBaudRateSupported

Change-Id: I22480b57d8c1b91ea7ea4a1da4ba3060e1a5f93f