rt2x00: Hardcode TX ack timeout and consume time

The calculated values for the ACK timeout and ACK
consume time are different then the values as
used by the Legacy drivers.

After testing from James Ledwith it appeared that
the calculated values caused a high amount of TX
failures, and the values from the Legacy drivers
were the most optimal to prevent TX failure due to
excessive retries.

The symptoms of this problem:
 - Rate control module always falls back to 1Mbs
 - Low throughput when bitrate was fixed

Possible side-effects (not confirmed but highly likely)
 - Problems with DHCP
 - Broken connections due to lack of probe response

This should fix at least:
Kernel bugzilla reports: [13362], [13009], [9273]
Fedora bugzilla reports: [443203]
but possible some additional bugs as well.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>