ionic: check for binary values in FW ver string

If the PCI connection is broken, reading the FW version string
will only get 0xff bytes, which shouldn't get printed.  This
checks the first byte and prints only the first 4 bytes
if non-ASCII.

Also, add a limit to the string length printed when a valid
string is found, just in case it is not properly terminated.

Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>