ide-scsi: remove superfluous BUG_ON() from idescsi_transfer_pc()

ide_set_handler() bugs on ->handler == NULL so no need to do it
in idescsi_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>