m68k/mac: Fix compiler warning in via_read_time()

The algorithm described in the comment compares two reads from the RTC but
the code actually reads once and compares the result to an uninitialized
value. This causes the compiler to warn, "last_result maybe used
uninitialized". Make the code match the comment, fix the warning and
perhaps improve reliability. Tested on a Quadra 700.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>