mac80211_hwsim: fix compiler warning on MIPS

The dividend in do_div() is expected to be an unsigned 64-bit integer,
which leads to the following warning when building for 32-bit MIPS:

  drivers/net/wireless/mac80211_hwsim.c: In function 'mac80211_hwsim_set_tsf':
  drivers/net/wireless/mac80211_hwsim.c:664:98: warning: comparison of distinct pointer types lacks a cast [enabled by default]
    data->bcn_delta = do_div(delta, bcn_int);

Since we care about the signedness of delta when adjusting tsf_offset
and bcm_delta, use the absolute value for the division and compare
the two timestamps to determine the sign.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>