libertas_sdio: handle spurious interrupts

Commit 06e8935f adds an IRQ handling
optimization for single-function SDIO cards like this one, but at the
same time exposes a small hardware bug.

During hardware init, an interrupt is generated with (apparently) no
source. Previously, mmc threw this interrupt away, but now (due to the
optimization), the mmc layer passes this onto libertas, before it is ready
(and before it has enabled interrupts), causing a crash.

Work around this hardware bug by registering the IRQ handler later and
making it capable of handling interrupts with no cause. The change that
makes the IRQ handler registration happen later actually eliminates
the spurious interrupt as well.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>