regmap-i2c: fix sparse warning in regmap_smbus_word_write_reg16

i2c_smbus_write_word_data() expects a plain u16, but cpu_to_le16()
returns __le16 (a sparse-restricted endian type), causing:

  drivers/base/regmap/regmap-i2c.c:340: sparse: incorrect type in
  argument 3 (different base types)
    expected unsigned short [usertype] value
    got restricted __le16 [usertype]

SMBus already defines byte ordering internally, so cpu_to_le16() is
wrong here. Replace it with a plain (u16) cast.

Fixes: bad4bd28abf4 ("regmap-i2c: add SMBus byte/word reg16 bus for adapters lacking I2C_FUNC_I2C")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202605161621.mY5zFh4D-lkp@intel.com/
Signed-off-by: Nishanth Sampath Kumar <nissampa@cisco.com>
Signed-off-by: Mark Brown <broonie@kernel.org>