Update check_equallogic.sh

Plus i agree with tatref's request to add a connectivity check.

See below :

- Before :
# ./check_equallogic.sh -C public -H bad_name -t disk
getaddrinfo: bad_name Name or service not known
snmpwalk: Unknown host (bad_name)
DISK OK 0 disks OK 0 disks spare
# echo $?
# 0

After :
# ./check_equallogic.sh -C public -H bad_name -t disk
getaddrinfo: bad_name Name or service not known
snmpwalk: Unknown host (bad_name)
UNKNOWN: check disk failed, please check your config.