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pcf8563.c bug

The RTC routines for the pcf8563 don't work properly, because certain 
operations on the read register values are done twice;

-pcf8563_readreg already masks out 'unused' bits, after which 'get_rtc_time' 
tries to check those bits (century bit and low voltage bit)

-'pcf8563_readreg' already decrements the month number, after that 
'get_rtc_time' does that again.

I think it should be OK if pcf8563_readreg just returns the raw values.

I'm using the RTC 8564 (EPSON), a chip that should be compatible with the 
pcf8563, judging by its datasheet.
However, it turns out that I have to add delays of up to 2ms (!) between 
rtc_writereg's, before it works.
This does not seem to be specified in the datasheets, and I'm wondering 
whether some problem with our design could cause this behaviour.
Apart from this, I2C communication seems to be working fine though.