libertas: fix IEEE PS mode in GSPI driver

The card firmware does not set the Command Download Ready interrupt bit
when IEEE PS mode is enabled, preventing the driver from sending
commands (such as the command to exit IEEE PS mode) since there is no
indication that the card is ready to accept commands.

This patch works around the problem by using the the TX Download Ready
bit in place of the Command Download Ready Bit while in IEEE PS mode.
TX Download Ready is set in IEEE PS mode.

Signed-off-by: Andrey Yurovsky <andrey@cozybit.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>