rt2x00: Optimize IV/EIV handling

IV and EIV belong to eachother and don't require
2 seperate fields. Instead they can logically be
merged into a single array with size 2.

With this approach we can simplify the code in
rt2x00crypto.c by using a single memcpy() when
copying the iv/eiv data. Additionally we can
move some code out of if-statements because the
if-statement would always be true.

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