net: dsa: qca8k: fix regmap bulk read/write methods on big endian systems

Commit c766e077 ("net: dsa: qca8k: convert to regmap read/write
API") introduced bulk read/write methods to qca8k's regmap.

The regmap bulk read/write methods get the register address in a buffer
passed as a void pointer parameter (the same buffer contains also the
read/written values). The register address occupies only as many bytes
as it requires at the beginning of this buffer. For example if the
.reg_bits member in regmap_config is 16 (as is the case for this
driver), the register address occupies only the first 2 bytes in this
buffer, so it can be cast to u16.

But the original commit implementing these bulk read/write methods cast
the buffer to u32:
  u32 reg = *(u32 *)reg_buf & U16_MAX;
taking the first 4 bytes. This works on little endian systems where the
first 2 bytes of the buffer correspond to the low 16-bits, but it
obviously cannot work on big endian systems.

Fix this by casting the beginning of the buffer to u16 as
   u32 reg = *(u16 *)reg_buf;

Fixes: c766e077 ("net: dsa: qca8k: convert to regmap read/write API")
Signed-off-by: Marek Behún <kabel@kernel.org>
Tested-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>