ath9k: Fix an operator typo in phy rate validation

This was not supposed to be a bitwise AND operation, but a check of
two separate conditions. Anyway, the old code happened to result in
the same behavior, so this is just changing the code to be easier to
understand and also to keep sparse from warning about dubious
operators.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>