brcmfmac: Transform compatible string for FW loading

The driver relies on the compatible string from DT to determine which
FW configuration file it should load. The DTS spec allows for '/' as
part of the compatible string. We change this to '-' so that we will
still be able to load the config file, even when the compatible has a
'/'. This fixes explicitly the firmware loading for
"solidrun,cubox-i/q".

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Hans deGoede <hdegoede@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200701112201.6449-1-matthias.bgg@kernel.org