USB: storage: sierra_ms: fix sysfs file attribute

A non-writable sysfs file shouldn't have writable attributes.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kevin Lloyd <klloyd@sierrawireless.com>
Cc: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>