mtd/chips: add missing set_current_state() to cfi_{amdstd,staa}_sync()

cfi_amdstd_sync() and cfi_staa_sync() call schedule() without changing task's
state appropriately.

In case of e.g.  chip->state == FL_ERASING, cfi_*_sync() will be busy-looping
either redundantly for a fixed interval of time (for SCHED_NORMAL tasks) or
possibly endlessly (for RT tasks and UP).

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>