usbip: fix usbip bind writing random string after command in match_busid

commit 544c4605 upstream.

usbip bind writes commands followed by random string when writing to
match_busid attribute in sysfs, caused by using full variable size
instead of string length.

Signed-off-by: Juan Zea <juan.zea@qindel.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>