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_force 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_force will be ignored.
So users with existing fwload_disable tunables will have the same
behavior.

If a user specifies both fwload_force 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.

share/man/man4/isp.4:
Document the new loader tunable.

sys/dev/isp/isp.c:
In isp_load_risc_flash(), changet the decision logic to
also consider ISP_CFG_FWLOAD_ONLY. Load the flash firmware
and get the version, so the user knows what it is, but if
the user set fwload_force, honor that. If the user didn't
set fwload_force, the behavior remains to select the newer
firmware version.

sys/dev/isp/isp_pci.c:
Add a new fwload_force tunable. Print out a warning if the
user sets both fwload_disable and fwload_force.

sys/dev/isp/ispvar.h:
Add a new ISP_CFG_FWLOAD_FORCE configuration bit.

Reviewed by: mav
MFC after: 1 week
Sponsored by: Spectra Logic
Differential Revision: https://reviews.freebsd.org/D45688

(cherry picked from commit 31354813f3c6e87532189be77c2f10a017c55472)