pci: Cosmetic cleanups to MSI/MSI-X routines

• Use unsigned integers for various variables. The count argument to the alloc method as well as the IRQ values used with the pcib_if.m methods should also be unsigned eventually.

• Use mallocarray to allocate arrays

• Use bool in a few places

Reviewed by: Krzysztof Galazka <krzysztof.galazka@intel.com>
Differential Revision: https://reviews.freebsd.org/D48889