libertas: Add auto deep sleep support for SD8385/SD8686/SD8688

Add timer based auto deep sleep feature in libertas driver which can be
configured using iwconfig command. This is tested on SD8688, SD8686 cards
with firmware versions 10.38.1.p25, 9.70.4.p0 respectively on 32-bit and 64-bit
platforms. Tests have been done for USB/CS cards to make sure that the patch
won't break USB/CS code. We didn't test the if_spi driver.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>