cxgbe(4): Allow negative values in hw.cxgbe.fw_install and take them to
mean that the driver should taste the firmware in the KLD and use that
firmware's version for all its fw_install checks.

The driver gets firmware version information from compiled-in values by
default and this change allows custom (or older/newer) firmware modules
to be used with the stock driver.

There is no change in default behavior.

MFC after: 1 week
Sponsored by: Chelsio Communications