scsi: st: osst: Remove negative constant left-shifts

Negative constant left-shift is undefined behaviour in the C standard, and
as such newer versions of clang (at least) warn against it. GCC supports it
for a long time, but it would be better to remove it and rely on defined
behaviour.

My understanding is "~(-1 << N)" in 2's complement is intended to generate
a bit pattern of zeroes ending with N '1' bits. The same can be achieved by
"(1 << N) - 1" in a well-defined way, so switch to it to remove the
warning.

Tested: building a kernel with generic SCSI tape, and checking basic
operations (mt status, mt eject) on a real LTO unit. Cannot test the osst
driver.

Signed-off-by: Iustin Pop <iustin@k1024.org>
Reviewed-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>