[IA64] hpsim: Fix check for overlong simscsi prefix.

When "strlen(s) > MAX_ROOT_LEN", it has already said to use the default
value, but in fact, it still use the input value.

If happens, next sprintf() for 'fname' in simscsi_queuecommand_lck()
may be memory overflow.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>