rt2x00: Move lna_gain calculation to config() callback

We can optimize lna calculation in IRQ context by
calculating most of the value during the config() callback
when most of the value is actually influenced.

This will be required later by rt2800pci and rt2800usb as
well, since they need the lna_gain value during config().

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