ide: add PC_FLAG_ZIP_DRIVE pc flag

Add PC_FLAG_ZIP_DRIVE pc flag, set it in idefloppy_do_request()
and check for it (instead of checking for IDEFLOPPY_FLAG_ZIP_DRIVE)
in idefloppy_transfer_pc().  This is a preparation for adding
generic ide_transfer_pc() helper.

There should be no functional changes caused by this patch.

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