ath5k: fix SIFS time handling

ath5k uses 8 usec as a sifs time, extracted from the initvals, whereas the
standard requires a sifs time of 10. The difference originates from the fact
that the SIFS register has an offset of 2 usec.
Fix the SIFS time definition to use the standard value of 10 usec and subtract
2 usecs when writing the SIFS register.

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