staging: nvec: allow modular build

The serio code can be a loadable module, which in turn means that nvec
also has to be a module, so mark it as "tristate" in Kconfig.

drivers/built-in.o: In function `nvec_mouse_remove':
drivers/staging/nvec/nvec_ps2.c:136: undefined reference to `serio_unregister_port'
drivers/built-in.o: In function `nvec_mouse_probe':
drivers/staging/nvec/nvec_ps2.c:126: undefined reference to `__serio_register_port'
drivers/built-in.o: In function `nvec_ps2_notifier':
drivers/staging/nvec/nvec_ps2.c:84: undefined reference to `serio_interrupt'
drivers/staging/nvec/nvec_ps2.c:91: undefined reference to `serio_interrupt'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>