rtc: omap: use rtc_write to access OMAP_RTC_OSC_REG

The RTC_OSC_REG register is 32-bit, but the useful information is found
in the 7 least significant bits (bits 7-31 are reserved). And in fact,
as you can see from the code, all read accesses are 8-bit, as well as
some writes. Let's make sure all writes are 8-bit. Moreover, in contexts
where consecutive reads / writes after the busy check must take place
within 15 us, it is better not to waste time on useless accesses.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210425145924.23353-1-dariobin@libero.it