staging: brcm80211: cleanup of shared memory related wrapper functions

- removed brcms_c_read_shm and brcms_c_write_shm functions.
- removed redundant argument check from brcms_c_copyto_shm function.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>