FDC: Fix buffer overflow (Hervé Poussineau)

In floppy controller, programming PIO writes which are more than one sector
long leads to a buffer overflow of the fdtrl->fifo[] array.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4293 c046a42c-6fe2-441c-8c8c-71466251a162
