m7-common: change usbdisk write permission

Users are not able to write to /storage/usbdisk because they are not
part of the media_rw (1023) group. Remove the write restriction.

Change-Id: I2ec50f3cc49154d2bfa60d8b5f1cfd54222e8a6e
Merge branch 'cm-11.0' into martincz