scsi: ufs-debugfs: modify "err_inj_scenario" debugfs entry

err_inj_scenario debugfs entry used to be a u32 entity.
This change modifies it to be a file entity, so reading it
(cat /d/.../err_inj_scenario) helps the user get useful information
about the enabled/disabled error scenarios.

Change-Id: Ia805e6f5357009752ddcace3ae4ed87061e48759
Signed-off-by: Yaniv Gardi <ygardi@codeaurora.org>