Add an isp(4) tunable to default to ispfw(4) firmware.

ispfw(4) recently gained firmware for Qlogic 27XX and 28XX FC controllers, and isp(4) now selects the newer of firmware in flash or in ispfw(4) to load for those controllers.

This differs from the previous behavior (which remains for older controllers), which was to always load the ispfw(4) firmware if it is available.

This adds a loader tunable, hint.isp.N.fwload_only to default to loading the ispfw(4) firmware, whether or not it is newer than the firmware in flash. This allows the user to always use the known firmware version included with the kernel.

Note that there is an existing fwload_disable tunable that tells the driver to always load the firmware from flash and ignore ispfw(4). If fwload_disable is set, fwload_only will be ignored. So users with existing fwload_disable tunables will have the same behavior.

If a user specifies both fwload_only and fwload_disable for the same controller, the isp(4) driver prints a warning message, and fwload_disable will be honored.

The user can see which firmware is active through the dev.isp.N.fw_version* sysctl variables.

Flash a 27XX or 28XX controller with firmware newer than the firmware available in ispfw(4). Set the fwload_only tunable, and see if the firmware in ispfw gets used.

Set both the fwload_only and fwload_disable tunables, and verify that the firmware in flash gets used.

Set neither tunable, and verify that the newer firmware gets used.