posix-timers: Define nanosleep not supported error separate

Define the conditional nanosleep not supported error value outside of
do_posix_clock_nonanosleep(). Preparatory patch for further cleanups. 

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <johnstul@us.ibm.com>
Tested-by: Richard Cochran <richard.cochran@omicron.at>
LKML-Reference: <20110201134417.643486574@linutronix.de>