libertas: fix power save issue in libertas_sdio module

The problem: "iwconfig ethX power on" returns error

The cause: "ps_supported" flag was never set for SD8385/8686

The fix: check firmware capabilities returned by GET_HW_SPEC command.
Set "ps_supported" to 1 if FW_CAPINFO_PS bit is on. This fix applies
to SDIO interface only.

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