ISDN: eicon: switch to use native bitmaps

Two arrays are clearly bit maps, so, make that explicit by converting to
bitmap API and remove custom helpers.

Note sig_ind() uses out of boundary bit to (looks like) protect against
potential bitmap_empty() checks for the same bitmap.

This patch removes that since:
1) that didn't guarantee atomicity anyway;
2) the first operation inside the for-loop is set bit in the bitmap
   (which effectively makes it non-empty);
3) group_optimization() doesn't utilize possible emptiness of the bitmap
   in question.

Thus, if there is a protection needed it should be implemented properly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>