USB: cxacru: add write-only sysfs attribute for modem configuration

The modem can be configured using CM_REQUEST_CARD_DATA_SET,
although CM_REQUEST_CARD_DATA_GET does not return any data.

Tested by setting the modulation (0x0a) option.

There is a list of parameters in the following archive,
but the meaning of many of them is not well documented:
  http://sourceforge.net/project/shownotes.php?release_id=301825



This source also indicates that the highest parameter set
is 0x4a but this varies by model so an arbitrary limit of
0x7f has been used (the index is a 32-bit integer).

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>