arm: tegra: comms: fix crash at dev attr change

arm: tegra: comms: fix crash at dev attr change

Check the state machine before poweron the modem by device
attribute change routine. Maintain the state machine to
avoid crash after writing state change to
 /sys/devices/platform/baseband_xmm_power/xmm_onoff

Also cancel workqueue at module unload to avoid
possible crash.

Bug 898008

Reviewed-on: http://git-master/r/72934

Change-Id: I0d8286774040f155c930ecb0bb69778de6c606ac
Signed-off-by: Vinayak Pane <vpane@nvidia.com>
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Reviewed-on: http://git-master/r/77744
Reviewed-by: Automatic_Commit_Validation_User
Rebase-Id: Reaea4fbd585c3f1cce910997ea17c7d8bfb544aa