ipw2100: fix postfix decrement errors

If i reaches zero, the loop ends, but the postfix decrement subtracts it to -1.
Testing for 'i == 0', later in the function, will not fulfill its purpose.

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>