Input: sirfsoc-onkey - allow modular build

CONFIG_INPUT may itself be a loadable module, but the sirf power key
driver is listed as 'bool', which makes it possible to select
a broken configuration with the driver built-in but the subsystem
not loaded. In this configuration, we get a link error:

drivers/input/built-in.o: In function `sirfsoc_pwrc_isr':
drivers/input/misc/sirfsoc-onkey.c:63: undefined reference to `input_event'
drivers/input/built-in.o: In function `sirfsoc_pwrc_isr':
include/linux/input.h:414: undefined reference to `input_event'
drivers/input/built-in.o: In function `sirfsoc_pwrc_probe':
drivers/input/misc/sirfsoc-onkey.c:132: undefined reference to `devm_input_allocate_device'
drivers/input/misc/sirfsoc-onkey.c:139: undefined reference to `input_set_capability'
drivers/input/misc/sirfsoc-onkey.c:161: undefined reference to `input_register_device'
drivers/input/built-in.o: In function `sirfsoc_pwrc_report_event':
drivers/input/misc/sirfsoc-onkey.c:48: undefined reference to `input_event'
drivers/input/built-in.o: In function `sirfsoc_pwrc_report_event':
include/linux/input.h:414: undefined reference to `input_event'
drivers/input/built-in.o:(.debug_addr+0x24): undefined reference to `input_event'
drivers/input/built-in.o:(.debug_addr+0xbc): undefined reference to `devm_input_allocate_device'
drivers/input/built-in.o:(.debug_addr+0x104): undefined reference to `input_set_capability'
drivers/input/built-in.o:(.debug_addr+0x128): undefined reference to `input_register_device'

This marks the driver as 'tristate' so it becomes possible to have
it in a loadable module, mainly to help with randconfig builds.

We also have to add a missing semicolon here, which ended up not
being needed in built-in mode because the following MODULE_DEVICE_TABLE
is an empty macro followed by another semicolon then.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>