staging: brcm80211: removed error string function

staging: brcm80211: removed error string function

Only fullmac used this functionality, in which a broadcom specific error
code was converted to a human readable string. Since this  functionality
is not needed, a function and accompanying data structures have been
removed.

Cc: devel@linuxdriverproject.org
Cc: linux-wireless@vger.kernel.org
Cc: Brett Rudley <brudley@broadcom.com>
Cc: Henry Ptasinski <henryp@broadcom.com>
Cc: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>