bus: mvebu-mbus: make iounmap() symmetric with ioremap()

make coccicheck complains:

  ./drivers/bus/mvebu-mbus.c:1113:2-8: ERROR: missing iounmap; ioremap on line 1106 and execution via conditional on line 1111

It took some staring but I don't think there is a problem because the
file global `mbus_state` is passed mvebu_mbus_common_init() as the
`mbus` parameter so `mbus_state.mbuswins_base` and `mbus->mbuswins_base`
are the same thing. But this is confusing for anyone reading the code
and one less complaint from coccicheck would be nice so lets fix it.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>