esp: cancel current request only if some request is in flight

This bug was introduced in 94d3f98a3f3caddd7875f9a11776daeb84962a7b:
scsi_cancel_io was checking if some request was pending before trying
to cancel it, while scsi_req_cancel always cancels the request.

This may lead to a crash of Qemu due to dereferencing a NULL pointer,
as exhibited by NetBSD 5.1 installer on MIPS Magnum emulation.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
