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

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, so this is safe.

Reviewed By: imp, kib, mav
Tested By:i dhw
Differential Revision: https://reviews.freebsd.org/D28023
Sponsored by: Netflix