i2c: i2c-bcm2835: match return type of wait_for_completion_timeout

return type of wait_for_completion_timeout is unsigned long not int. as
time_left is used for wait_for_completion_timeout exclusively here its
type is simply changed to unsigned long.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>