asm-generic: make get_rtc_time overridable

asm-generic: make get_rtc_time overridable

Evidently, set_rtc_time is supposed to be overridable
by architectures that define their own version, but
unfortunately, get_rtc_ss would in that case still
use the generic version.

This makes get_rtc_ss call the real set_rtc_time
to let architectures define their own version.
The change should fix the "Extended RTC operation"
on Alpha, which uses the incorrect get_rtc_ss
call. It also allows PowerPC to use the asm-generic/rtc.h
file in the future.

Cc: Richard Henderson <rth@twiddle.net>
Cc: linux-alpha@vger.kernel.org
Cc: Tom Rini <trini@mvista.com>
Cc: rtc-linux@googlegroups.com
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Paul Gortmaker <p_gortmaker@yahoo.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>