usb: acm: fix the computation of the number of data bits

The current code assumes that CSIZE is 0000060, which appears to be
wrong on some arches (such as powerpc).

Signed-off-by: Nicolas Boullis <nboullis@debian.org>
Acked-by: Oliver Neukum <oneukum@suse.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>