PWM channel data was set to NULL before freeing it. This caused the
following kernel warning while unexporting the channel. Set the channel
data to NULL after freeing it.

[   70.495000] WARNING: CPU: 0 PID: 1 at drivers/base/devres.c:805 pwm_put+0x48/0x80()
[   70.505000] Modules linked in:
[   70.505000] CPU: 0 PID: 1 Comm: sh Not tainted 3.12.0-rc6-next-20131024-00012-gd4aec04-dirty #58
[   70.515000] [<c0014ddc>] (unwind_backtrace+0x0/0xf4) from [<c0011784>] (show_stack+0x10/0x14)
[   70.525000] [<c0011784>] (show_stack+0x10/0x14) from [<c03a3cbc>] (dump_stack+0x7c/0xb0)
[   70.530000] [<c03a3cbc>] (dump_stack+0x7c/0xb0) from [<c001de68>] (warn_slowpath_common+0x6c/0x88)
[   70.540000] [<c001de68>] (warn_slowpath_common+0x6c/0x88) from [<c001df20>] (warn_slowpath_null+0x1c/0x24)
[   70.550000] [<c001df20>] (warn_slowpath_null+0x1c/0x24) from [<c01d1af8>] (pwm_put+0x48/0x80)
[   70.560000] [<c01d1af8>] (pwm_put+0x48/0x80) from [<c01d21c8>] (pwm_unexport_store+0x94/0xac)
[   70.565000] [<c01d21c8>] (pwm_unexport_store+0x94/0xac) from [<c010991c>] (sysfs_write_file+0x148/0x1d8)
[   70.575000] [<c010991c>] (sysfs_write_file+0x148/0x1d8) from [<c00b2a80>] (vfs_write+0xb4/0x1a0)
[   70.585000] [<c00b2a80>] (vfs_write+0xb4/0x1a0) from [<c00b3068>] (SyS_write+0x3c/0x78)
[   70.595000] [<c00b3068>] (SyS_write+0x3c/0x78) from [<c000e2e0>] (ret_fast_syscall+0x0/0x30)
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>