rt2x00: Move firmware checksumming to driver

rt2x00lib depended on 2 crc algorithms because rt61/rt73
use a different algorithm then rt2800. This means that
even when only 1 algorithm was needed, the dependency was
still present for both.
By moving the checksum generation to the driver we can clean
up 2 annoying flags (which indicated which checksum was required)
and move the dependency to where it belongs: the driver.

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