Input: dm355evm_keys - use threaded IRQs

Convert the dm355evm keys driver to use IRQ threading instead of
a private workqueue.  IRQ threads were added to Linux after this
driver was written, and in this case fit what the driver needs.
(Although the non-shared thread costs more runtime memory.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>