PM: hibernate: fix sparse warnings

When building the kernel with sparse enabled 'C=1' the following
warnings shows up:

kernel/power/swap.c:390:29: warning: incorrect type in assignment (different base types)
kernel/power/swap.c:390:29:    expected int ret
kernel/power/swap.c:390:29:    got restricted blk_status_t

This is due to function hib_wait_io() returns a 'blk_status_t' which is
a bitwise u8. Commit 5416da01 ("PM: hibernate: Remove
blk_status_to_errno in hib_wait_io") seemed to have mixed up the return
type. However, the 4e4cbee9 ("block: switch bios to blk_status_t")
actually broke the behaviour by returning the wrong type.

Rework so function hib_wait_io() returns a 'int' instead of
'blk_status_t' and make sure to call function
blk_status_to_errno(hb->error)' when returning from function
hib_wait_io() a int gets returned.

Fixes: 4e4cbee9 ("block: switch bios to blk_status_t")
Fixes: 5416da01 ("PM: hibernate: Remove blk_status_to_errno in hib_wait_io")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>