HID: hid-steam: Better handling of serial number length

The second byte of the GET_STRING_ATTRIB report is a length, so we should set
the size of the buffer to be the size we're actually requesting, and only
reject the reply if the length out is nonsensical.

Signed-off-by: Vicki Pfau <vi@endrift.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>