ath5k: ignore the return value of ath5k_hw_noise_floor_calibration

Noise floor calibration occasionally fails on Atheros hardware.
This is not fatal and can happen if there's simply too much
noise on the air. Ignoring the calibration error is the right
thing to do here, because when the error is ignored, the hardware
will still work, whereas if the error causes the driver to bail out
of a bigger configuration function and does not configure the tx
queues or the IMR (as is the case in reset.c), the hw no longer
works properly until the next reset.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>