[ARM] rtc-pcf8583: don't use BCD_TO_BIN/BIN_TO_BCD

Both BCD_TO_BIN(x) and BIN_TO_BCD(x) have an unexpected side-effect -
not only do they return the value as expected, they _modify_ their
argument in the process.

Let's play it safe and avoid these macros.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>