related registers for half/quarter channels

Initialize the the clock-to-TSF field of AR_USEC and the SIFS and EIFS time
registers based on the clock rate instead of relying on initvals.
With those changes, some of the hardcoded AR9287 1.3+ specific overrides
can be dropped.

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