mmc: core: reduce erase event timeout

mmc: core: reduce erase event timeout

We set wrong discard timeout limitation for erase event
which exceed that block layer does. And jbd warning
can be observed like blow:

INFO: task update_binary:151 blocked for more than 10 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
update_binary   D ffffffc000085674     0   151    134 0x00400008
Call trace:
[<ffffffc000085674>] __switch_to+0x80/0x8c
[<ffffffc00082cb28>] __schedule+0x4e0/0x6f4
[<ffffffc00082cda0>] schedule+0x64/0x70
[<ffffffc00082aea0>] schedule_timeout+0x28/0x250
[<ffffffc00082d2ac>] io_schedule_timeout+0x6c/0xa8
[<ffffffc00082d37c>] wait_for_common_io.constprop.111+0x94/0x114
[<ffffffc00082d41c>] wait_for_completion_io+0xc/0x18
[<ffffffc0002ffb08>] blkdev_issue_discard+0x1e0/0x214
[<ffffffc000300630>] blkdev_ioctl+0x2d0/0x708
[<ffffffc00030d52c>] compat_blkdev_ioctl+0x464/0x5ec
[<ffffffc0001bdf0c>] compat_sys_ioctl+0x104/0x1e8

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>