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

commit 301a29da6e891e7eb95c843af0ecdbe86d01f723 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>