[CHAR] ds1286: Fix handling of seconds in RTC_ALM_SET ioctl.

 o Fix use of uninitialized variable sec.
 o Make the RTC_ALM_SET ioctl return -EINVAL for non-zero seconds - the
   DS1286 has no second field for the alarm time.
 o Replace the obscure BIN_TO_BCD macro with BIN2BCD.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>