ipw2200: Check for -1 INTA in tasklet too.

This is an attempt to fix a long standing open bug:
http://bugzilla.intellinuxwireless.org/show_bug.cgi?id=1334

The interrupt handler checks for INTA being -1, apparently that means that the
hardware is gone. But the interrupt handler defers actual interrupt processing
to a tasklet. By the time the tasklet is run and checks INTA again, the
hardware might be gone and INTA be -1, which confuses the driver because all
event bits are set.

The patch applies to 2.6.37.

Signed-off-by: Indan Zupancic <indan@nul.nu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>