MdePkg SmmIoLib: Use NULL pointer check instead of useless Status check

MdePkg SmmIoLib: Use NULL pointer check instead of useless Status check

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=587

The Status check in "if (!EFI_ERROR (Status))" condition is useless,
it should be NULL pointer check. And this patch also fixes a typo
"continous" to "continuous".

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>