iflib: Simplify iflib_legacy_setup

Follow the pattern from iflib_irq_alloc_generic function and use
iflib_fast_intr as a handler for RX only interrupts.

Also remove some intermediate variables and use consistent
way for referencing queue's structures.

Signed-off-by: Krzysztof Galazka <krzysztof.galazka@intel.com>

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D46061

(cherry picked from commit 36a001923836e280e750b76947b8705fcc47b0b7)