sdcard: fix usage for -w and -s options

Summary: Change-Id: I0ef4299d2ccf99a18970f02d4bd4e7c28dc5335e

Test Plan: verify with ntfs/fat32 OTG support

Reviewers: synergy, randall.rushing, donald.munn, raymanfx

Differential Revision: http://review.codefirex.com/D244