rtc: isl1208: Add isl1208_set_xtoscb()

As per the HW manual, set the XTOSCB bit as follows:

If using an external clock signal, set the XTOSCB bit as 1 to
disable the crystal oscillator.

If using an external crystal, the XTOSCB bit needs to be set at 0
to enable the crystal oscillator.

Add isl1208_set_xtoscb() to set XTOSCB bit based on the clock-names
property. Fallback is enabling the internal crystal oscillator.

While at it, introduce a variable "sr" for reading the status register
in probe() as it is reused for writing and also remove the unnecessary
blank line.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20230623140948.384762-10-biju.das.jz@bp.renesas.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>