Changed Tx Pwr attenator range to 45dB

This  is now  half way  between  the recent  change to  30dB when  the
formula was corrected  to voltage attenuation from power  and prior to
that.

Also corrected  formula for  the SoundOutput::attenuation  property to
use the correct voltage attenuation calculation.

git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@5579 ab8295b8-cf94-4d9e-aec4-7959e3be5d79

mainwindow.ui

@@ -1257,7 +1257,7 @@
            <string>Adjust Tx audio level</string>
           </property>
           <property name="maximum">
-           <number>300</number>
+           <number>450</number>
           </property>
           <property name="value">
            <number>0</number>

soundout.cpp

@@ -140,7 +140,7 @@ void SoundOutput::stop ()
 
 qreal SoundOutput::attenuation () const
 {
-  return -(10. * qLn (m_volume) / qLn (10.));
+  return -(20. * qLn (m_volume) / qLn (10.));
 }
 
 void SoundOutput::setAttenuation (qreal a)