tifm_sd: Switch software timeout handler from work_struct to timer

Two changes are introduced to software timeout handler in order to simplify
its management:
1. The implementation is switched from work_struct to timer
2. Previously, software timeout was rearmed with each interrupt. Now,
current request must complete entirely within timeout interval.

Signed-off-by: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>