NetworkPkg/IScsiDxe: Fix the incorrect error handling in DriverEntryPoint

NetworkPkg/IScsiDxe: Fix the incorrect error handling in DriverEntryPoint

Currently, error handling in IScsiDriverEntryPoint is incorrect. For
example, if IScsiCreateAttempts() return error due to the limited max
variable size, iSCSI will not unload the configuration entries.

Cc: Zhang Lubo <lubo.zhang@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Zhang Lubo <lubo.zhang@intel.com>