isci: use maxphys rather than 128KB to size s/g list

[gallatin]

Summary

In the conversion into a tunable, we converted the
size of the s/g list used by the driver to be based
off of a hardcoded size of 128k rather than maxphys,
this caused performance problems for us. Revert this
to use the maxphys tunable.

Note that this constant is used to size dynamically allocated
things, and not static data structs, and is not constrained
by controller hardware limits, so this is safe.

[imp]

[dhw]

I tested this (at work); it works in our workload (and avoids a severe performance regression).