USB: serial: mos7840: fix status-register error handling

Add missing transfer-length sanity check to the status-register
completion handler to avoid leaking bits of uninitialised slab data to
user space.

Fixes: 3f542974 ("USB: Moschip 7840 USB-Serial Driver")
Cc: stable <stable@vger.kernel.org>     # 2.6.19
Signed-off-by: Johan Hovold <johan@kernel.org>