mwifiex: debugfs: correct histogram spacing, formatting

Currently, snippets of this file look like:

rx rates (in Mbps): 0=1M   1=2M2=5.5M  3=11M   4=6M   5=9M  6=12M
7=18M  8=24M  9=36M  10=48M  11=54M12-27=MCS0-15(BW20) 28-43=MCS0-15(BW40)
44-53=MCS0-9(VHT:BW20)54-63=MCS0-9(VHT:BW40)64-73=MCS0-9(VHT:BW80)
...
noise_flr[--96dBm] = 22
noise_flr[--95dBm] = 149
noise_flr[--94dBm] = 9
noise_flr[--93dBm] = 2

We're missing some spaces, and we're adding a minus sign ('-') on values
that are already negative signed integers.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>