iio: magnetometer: mmc35240: Fix crash in pm suspend

iio: magnetometer: mmc35240: Fix crash in pm suspend

We must set i2c client private data at probe in order to
correctly retrieve it in pm suspend/resume, preventing
the following crash:

[ 321.790582] PM: Syncing filesystems ... done.
[ 322.364440] PM: Preparing system for mem sleep
[ 322.400047] PM: Entering mem sleep
[ 322.462178] BUG: unable to handle kernel NULL pointer dereference at 0000036c
[ 322.469119] IP: [<80e0b3d2>] mmc35240_suspend+0x12/0x30
[ 322.474291] *pdpt = 000000002fd6f001 *pde = 0000000000000000
[ 322.479967] Oops: 0000 1 PREEMPT SMP
[ 322.496516] task: a86d0df0 ti: a8766000 task.ti: a8766000
[ 322.570744] Call Trace:
[ 322.573217] [<80c0d2d1>] pm_generic_suspend+0x21/0x30
[ 322.578284] [<80d042ab>] i2c_device_pm_suspend+0x1b/0x30

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>