Make firmware caching on startup optional, and make it default.

When the option is not selected and PM_SLEEP is configured, then
cache firmware in the suspend pm_notifier. This configuration saves
about 64k RAM in normal use, but can lead to a situation where the
driver is configured to use a different firmware.

Signed-off by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>