scsi: 3w-sas: 3w-9xxx: Use unsigned char for cdb

Clang warns a few times:

drivers/scsi/3w-sas.c:386:11: warning: implicit conversion from 'int' to
'char' changes value from 128 to -128 [-Wconstant-conversion]
        cdb[4] = TW_ALLOCATION_LENGTH; /* allocation length */
               ~ ^~~~~~~~~~~~~~~~~~~~

Update cdb's type to unsigned char, which matches the type of the cdb
member in struct TW_Command_Apache.

Link: https://github.com/ClangBuiltLinux/linux/issues/158


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Adam Radford <aradford@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>