memstick: r592: fix build warnings for !PM_SLEEP

When PM_SLEEP is not enabled, the r592_clear_interrupts() function is
never used.  If so, don't build it to prevent a compiler warning.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>