clk: si521xx: Fix regmap write accessor

Rework the write operation such that the Byte Count register is written with
a single raw i2c write outside of regmap using transfer which does specify
the number of bytes to be transfered, one in this case, and which makes the
expected subsequent write transfer look like address+register+data, and then
make use of this method. Without this change, the Byte Count register write
in probe() would succeed as it would provide the byte count as part of its
write payload, but any subsequent writes would fail due to this Byte Count
register programming. Such failing writes happens e.g. during resume, when
restoring the regmap content.

Fixes: edc12763

 ("clk: si521xx: Clock driver for Skyworks Si521xx I2C PCIe clock generators")
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230831181656.154750-2-marex@denx.de
Signed-off-by: Stephen Boyd <sboyd@kernel.org>