staging: fsl-mc: fix warning in DT ranges parser

The fsl-mc-bus driver in staging contains a copy of the standard
'ranges' property parsing algorithm with a hack to treat a missing
property the same way as an empty one. This code produces false-positive
warnings for me in an allmodconfig build:

drivers/staging/fsl-mc/bus/fsl-mc-bus.c: In function 'fsl_mc_bus_probe':
drivers/staging/fsl-mc/bus/fsl-mc-bus.c:645:6: error: 'mc_size_cells' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/staging/fsl-mc/bus/fsl-mc-bus.c:682:8: error: 'mc_addr_cells' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/staging/fsl-mc/bus/fsl-mc-bus.c:644:6: note: 'mc_addr_cells' was declared here
drivers/staging/fsl-mc/bus/fsl-mc-bus.c:684:8: error: 'paddr_cells' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/staging/fsl-mc/bus/fsl-mc-bus.c:643:6: note: 'paddr_cells' was declared here

To avoid the warnings, I'm simplifying the argument handling to pass
the number of valid ranges in the property as the function return code
rather than passing it by reference. With this change, gcc can see that
we don't evaluate the cell numbers for an missing ranges property.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>