Allow fragmentation/RTS threshold to be disabled explicitly

Allow fragmentation/RTS threshold to be disabled explicitly

hostapd configuration parameters fragm_threshold and rts_threshold were
documented to disable the threshold with value -1 and not change driver
configuration if the parameter is not included. However, -1 was mapped
into not changing the driver value, so the explicit disabling part did
not work.

Replace the default values for these to be -2 so that explicitly set
configuration value -1 can be distinguished from the case of not
including the parameter. Map the -1 value to a driver request to disable
the threshold. Ignore any error from this operation just in case to
avoid breaking functionality should some drivers not accept the (u32) -1
value as a threshold value request to disable the mechanism.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>