ACPI: EC: Make the GPE storm threshold a module parameter

The Linux EC driver includes a mechanism to detect GPE storms,
and switch from interrupt-mode to polling mode.  However, polling
mode sometimes doesn't work, so the workaround is problematic.
Also, different systems seem to need the threshold for detecting
the GPE storm at different levels.

ACPI_EC_STORM_THRESHOLD was initially 20 when it's created, and
was changed to 8 in 2.6.28 commit 06cf7d3c "ACPI: EC: lower interrupt storm
threshold" to fix kernel bug 11892 by forcing the laptop in that bug to
work in polling mode. However in bug 45151, it works fine in interrupt
mode if we lift the threshold back to 20.

This patch makes the threshold a module parameter so that user has a
flexible option to debug/workaround this issue.

The default is unchanged.

This is also a preparation patch to fix specific systems:
	https://bugzilla.kernel.org/show_bug.cgi?id=45151



Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>
cc: stable@vger.kernel.org