mwifiex: control WLAN and bluetooth coexistence modes

By default our chip will be in spatial coexistence mode.
This patch adds a provision to change it to timeshare mode
via debugfs command.

Enable timeshare coexistence mode
   echo 1 > /sys/kernel/debug/mwifiex/mlan0/timeshare_coex

Go back to spacial coexistence mode
   echo 0 > /sys/kernel/debug/mwifiex/mlan0/timeshare_coex

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>