ixl(4): Fix queue MSI and legacy IRQ rearming

When MSI or legacy interrupt is used driver controls wheter
queues can trigger an interrupt with the Interrupt Linked List.
While processing traffic first index of the list is set to EOL
value to stop queues from triggering interrupts. This index was
not reset to the correct value when driver attempted to re-enable
interrupts from queues, what prevented driver from processing any
traffic. Fix that by setting correct first index in the
ixl_if_enable_intr function.

While at that fix the comments style and make ixl_if_enable_intr
and ixl_if_disable_intr more consistent.

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

PR: 288077
Suggested by: Mike Belanger <mibelanger@qnx.com>
Approved by: kbowling (mentor), erj (mentor)
Tested by: gowtham.kumar.ks_intel.com,
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D51331

(cherry picked from commit 6f41c1fc39d9fa9db989a7b4f325c3ab85b8fb45)