iflib: mark irq allocation name parameter as constant

The *name parameter passed to iflib_irq_alloc_generic and
iflib_softirq_alloc_generic is never modified. Many places in code pass
string literals and thus should not be modified.

Mark the *name parameter as a const char * instead, so that we enforce
that the name is not modified before passing to bus_describe_intr()

Submitted by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed by: kmacy
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D15343