Axe MINIMUM_MSI_INT.

Just allow MSI interrupts to always start at the end of the I/O APIC
pins. Since existing machines already have more than 255 I/O APIC
pins, IRQ 255 is no longer reliably invalid, so just remove the
minimum starting value for MSI.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D17991