staging: nvec: remove duplicated const

Sparse checking warning:
"drivers/staging/nvec/nvec_ps2.c:172:14: warning: duplicate const".
Remove the duplicated const to fix the warning.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Acked-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>