Revert iwn channel-argument handling of set_txpower() to r201822

Pass the channel argument as a real argument and not through
RXON. The RXON version seems to have problem with both the older
228.57.2.23 firmware and the latest 228.61.2.24 version resulting
in device initialization errors like:

wpa_supplicant[2928]: Failed to initiate AP scan.
kernel: firmware error log:
kernel: error type      = "SYSASSERT" (0x00000005)
kernel: program counter = 0x0000147C
kernel: source line     = 0x0000058B
kernel: error data      = 0x0000058B00000000
kernel: branch link     = 0x0000145A00001492
kernel: interrupt link  = 0x000006DE00000000
kernel: time            = 7310
...
kernel: iwn0: iwn_config: could not set TX power
kernel: iwn0: iwn_init_locked: could not configure device, error 35

By passing the current channel to hal->set_txpower() the firmware
error is fixed, at least for the 4965 chipset of my Thinkpad and
the ones tested by Bernhard.

Submitted by: Bernhard Schmidt <bschmidt@techwires.net>
Approved by: rpaulo