mwifiex: set wiphy params only once

RTS threshold, fragmentation threshold are per device properties.
Setting them on any interface would be reflected for all other
interfaces as well. This patch removes unnesessary command download
per interface.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>