mwifiex: rename function mwifiex_is_ba_stream_avail

The old function name sounds like checking for existing BA
stream. The function actually checks if we have room for
creating new BA stream or not.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>