staging: sbe-2t3e3: fix sparse warnings

warnings:
drivers/staging/sbe-2t3e3/netdev.c:24:5: warning: symbol 't3e3_ioctl' was not declared. Should it be static?
drivers/staging/sbe-2t3e3/netdev.c:85:5: warning: symbol 't3e3_open' was not declared. Should it be static?
drivers/staging/sbe-2t3e3/netdev.c:100:5: warning: symbol 't3e3_close' was not declared. Should it be static?

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>