MdeModulePkg/UsbBusDxe: Add missing "return NULL" in UsbCreateDesc()

MdeModulePkg/UsbBusDxe: Add missing "return NULL" in UsbCreateDesc()

When (Len < Offset) is TRUE, indicating the data to visit is beyond
the boundary, the error message is printed but the function doesn't
return NULL.

It's a typo when modifying the commit 4c034bf62.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>