- When creating the DMA tag for user data, don't ask for more segments

- When creating the DMA tag for user data, don't ask for more segments
  than required for handling MAXPHYS and report the resulting maximum
  I/O size to CAM instead of implicitly limiting it to DFLTPHYS.
- Move the variables of sym_action2() out of nested scope as required
  by style(9) and remove extraneous curly braces.
- Replace a magic value for PCIR_COMMAND with the appropriate macro.
- Use DEVMETHOD_END.
- Use NULL instead of 0 for pointers.

Tested with a HBA donated by wilko.

MFC after:	3 days