mfd: altera-sysmgr: Fix device leak on sysmgr regmap lookup

mfd: altera-sysmgr: Fix device leak on sysmgr regmap lookup

Make sure to drop the reference taken to the sysmgr platform device when
retrieving its driver data.

Note that holding a reference to a device does not prevent its driver
data from going away.

Fixes: f36e789a1f8d ("mfd: altera-sysmgr: Add SOCFPGA System Manager")
Cc: stable@vger.kernel.org	# 5.2
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>