staging: brcm80211: reorg brcms_c_validboardtype for clarity

The structure of the function made it difficult to understand, so reorganize it
to make it clearer.

Reported-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>
Signed-off-by: Henry Ptasinski <henryp@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>