rt2x00: initialize last_reset

Initialize last_reset variable to INITIAL_JIFFIES, otherwise it is not
possible to test H/W reset for first 5 minutes of system run.

Fixes: e403fa31 ("rt2x00: add restart hw")
Reported-and-tested-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>