drivers: shs: fix deadlock caused between generic netlink and rtnl locks

Fix for a deadlock seen when trying to register the shs generic netlink family
inside of the callback context of rtnl_netlink. Instead of running in the
notifier's context, the generic netlink registration is moved to initialization
of the kernel module. Fixes the following lock contention scenario:

[ 3302.102281] Call trace:
[ 3302.102332] __switch_to+0x108/0x118
[ 3302.102357] __schedule+0x8fc/0xcd8
[ 3302.102368] schedule_preempt_disabled+0x7c/0xa8
[ 3302.102384] __mutex_lock+0x444/0x660
[ 3302.102392] __mutex_lock_slowpath+0x10/0x18
[ 3302.102399] mutex_lock+0x30/0x38 mutex_lock(&rtnl_mutex);
[ 3302.102422] rtnl_lock+0x14/0x20 rtnl_lock
[ 3302.102448] nl80211_pre_doit+0x28/0x1a0
[ 3302.102465] genl_rcv_msg+0x3a4/0x408
[ 3302.102473] netlink_rcv_skb+0xa8/0x120
[ 3302.102481] genl_rcv+0x30/0x48 down_read(&cb_lock);
[ 3302.102487] netlink_unicast+0x1ec/0x290
[ 3302.102496] netlink_sendmsg+0x2ec/0x348

[ 3302.102609] Call trace:
[ 3302.102615] __switch_to+0x108/0x118
[ 3302.102624] __schedule+0x8fc/0xcd8
[ 3302.102630] schedule+0x70/0x90
[ 3302.102638] rwsem_down_write_failed+0x2bc/0x3c8
[ 3302.102644] down_write+0x4c/0x50
[ 3302.102652] genl_register_family+0xb4/0x650 down_write(&cb_lock);
[ 3302.102818] rmnet_shs_wq_genl_init+0x1c/0x38 [rmnet_shs]
[ 3302.102847] rmnet_shs_wq_init+0x218/0x328 [rmnet_shs]
[ 3302.102873] rmnet_shs_dev_notify_cb+0x378/0x3e0 [rmnet_shs]
[ 3302.102892] raw_notifier_call_chain+0x3c/0x68
[ 3302.102909] register_netdevice+0x374/0x560
[ 3302.102934] rmnet_vnd_newlink+0x6c/0xe8
[ 3302.102942] rmnet_newlink+0x9c/0x198
[ 3302.102950] rtnl_newlink+0x648/0x7b0
[ 3302.102960] rtnetlink_rcv_msg+0x270/0x388 mutex_lock(&rtnl_mutex);

Change-Id: Ib71de0cb4617477cab40a7f42154584765e30c2b
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>