xen/grant_table: cleanup max_nr_grant_frames()

[ehem_freebsd_m5p.com]

Summary

This is no more or less than returning the smaller of two values. Since
this is what min() does, use that to shrink max_nr_grant_frames() down
to the single line.

[mhorne]

Looks fine to me. I was wondering if __max_nr_grant_frames() would be evaluated twice, but it seems not as min() is a static inline function, unlike the macro MIN().