math:p_ln: Use M_LN2 instead of 'magic number'
math:p_ln: Use M_LN2 instead of 'magic number'

Signed-off-by: Ola Jeppsson <793f970c52ded1276b9264c742f19d1888cbaf73@adapteva.com>

debug-de-su-ka/pal,aolofsson/pal,debug-de-su-ka/pal,RafaelRMA/pal,eliteraspberries/pal,8l/pal,parallella/pal,olajep/pal,aolofsson/pal,mateunho/pal,eliteraspberries/pal,olajep/pal,8l/pal,olajep/pal,Adamszk/pal3,eliteraspberries/pal,parallella/pal,aolofsson/pal,mateunho/pal,RafaelRMA/pal,olajep/pal,8l/pal,mateunho/pal,RafaelRMA/pal,Adamszk/pal3,mateunho/pal,aolofsson/pal,parallella/pal,debug-de-su-ka/pal,parallella/pal,8l/pal,debug-de-su-ka/pal,mateunho/pal,debug-de-su-ka/pal,eliteraspberries/pal,parallella/pal,Adamszk/pal3,RafaelRMA/pal,Adamszk/pal3,eliteraspberries/pal