rt2x00: Gather channel information in structure

Channel information which is read from EEPROM should
be read into an array containing per-channel information.
This removes the requirement of multiple arrays and makes
the channel handling a bit cleaner and easier to expand.

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