mISDN: improve bitops usage

This improves bitops usages in several points:

- Convert u64 to a proper bitmap declaration.  This enables to remove
  superfluous typecasting from 'u64' to 'unsigned long *'.

- Convert superfluous atomic bitops to non atomic bitops.  The bitmap
  is allocated on the stack and it is not accessed by any other threads,
  so using atomic bitops is not necessary.

- Use find_next_zero_bit and find_next_zero_bit instead of calling
  test_bit() for each bit.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>