staging: most: i2c: shorten lifetime of IRQ handler

Currently the IRQ handler used for the rx channel lives between the
functions i2c_probe and i2c_remove. This patch shortens the lifetime
and keeps the handler alive only between the functions configure_channel
and poison_channel.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>