i2c: ismt: fix type of return var of wait_for_completion_timeout

return type of wait_for_completion_timeout is unsigned long not int. As
ret is in used for other calls a new appropriately typed variable timeout
is added to handle wait_for_completion_timeout

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>