mmc: block: stop bkops during shutdown

[   42.885198] SysRq : Emergency Remount R/O
[   43.003988] mmc0: mmc_bus_shutdown: invoked ###
[   43.033289] mmc0: mmc_bus_shutdown: done ###
[   43.118486] mmc0: sdhci_request: sending cmd8 after bus shutdown !!!
[   43.147955] [<c0659ca4>] (sdhci_request+0x4c/0x260) from
				[<c063ee24>] (mmc_start_request+0x27c/0x2b0)
[   43.157152] [<c063ee24>] (mmc_start_request+0x27c/0x2b0) from
				[<c063eec4>] (mmc_wait_for_req+0x6c/0x13c)
[   43.166618] [<c063eec4>] (mmc_wait_for_req+0x6c/0x13c) from
				[<c0645ed4>] (mmc_send_cxd_data+0xe0/0x114)
[   43.175991] [<c0645ed4>] (mmc_send_cxd_data+0xe0/0x114) from
				[<c0645f24>] (mmc_send_ext_csd+0x1c/0x20)
[   43.185277] [<c0645f24>] (mmc_send_ext_csd+0x1c/0x20) from
				[<c063df28>] (mmc_read_bkops_status+0x98/0xdc)
[   43.194823] [<c063df28>] (mmc_read_bkops_status+0x98/0xdc) from
				[<c06417ac>] (mmc_start_bkops+0x168/0x324)
[   43.204461] [<c06417ac>] (mmc_start_bkops+0x168/0x324) from
				[<c01a9584>] (process_one_work+0x200/0x400)
[   43.213835] [<c01a9584>] (process_one_work+0x200/0x400) from
				[<c01a9934>] (worker_thread+0x184/0x2a4)
[   43.223039] [<c01a9934>] (worker_thread+0x184/0x2a4) from
				[<c01ae010>] (kthread+0x80/0x90)
[   43.231286] [<c01ae010>] (kthread+0x80/0x90) from
				[<c0106c4c>] (kernel_thread_exit+0x0/0x8)
[   45.648206] wcnss crash shutdown 0
[   45.650599] Rebooting in 5 seconds..

In the above log, it is seen that the bkops thread runs after
mmc_bus_shutdown is completed and sends CMD8. Hence, cancel
bkops work during shutdown to prevent sending commands after
mmc bus is shutdown.

CRs-fixed: 593813
Change-Id: I3e8e9eb228bd03603d04561eb478d60664f50aaa
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>