Input: tegra-kbc - tighten locking

Take spinlock when entering ISR and timer routine to ensure that we do not
race while enabling/disabling FIFO interrupts.

Also we do not need to take teh spinlock in tegra_kbc_startremove() since
interrupt is completely disabled.

Tested-by: Rakesh Iyer <riyer@nvidia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>