pvscsi: Explicitly mask out CTS_SAS_VALID_SPEED bit

This driver prefers connection speed over sas port speed. On stable/13
branch the stack-allocated CCB is not cleared thus the cam layer may
report weird speed on boot.

da0: <VMware Virtual disk 2.0> Fixed Direct Access SPC-4 SCSI device
da0: 4294967.295MB/s transfers

-current and stable/14 have the change [1] which clears stack-allocated
CCB thus are not affected, but I want -current and stable/14 to have this
fix in to reduce drift between branches.

1. ec5325dbca62 cam: make sure to clear even more CCBs allocated on the stack

Reviewed by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D48438