ide-floppy: merge idefloppy_transfer_pc() and idefloppy_transfer_pc1()

* Check IDEFLOPPY_FLAG_ZIP_DRIVE flag in idefloppy_transfer_pc1()
  and skip idefloppy_transfer_pc2()-phase if the flag is not set.

* Always use idefloppy_transfer_pc1() in idefloppy_issue_pc()
  and remove no longer needed idefloppy_transfer_pc().

There should be no functional changes caused by this patch.

Cc: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>