i2c: mpc: Drop unused variable

Fix the following build error.

Error log:
drivers/i2c/busses/i2c-mpc.c: In function 'mpc_i2c_setup_512x':
drivers/i2c/busses/i2c-mpc.c:310:20: error: unused variable 'pval'

Fixes: 9d178e00

 ("i2c: mpc: Use of_property_read_reg() to parse "reg"")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wolfram Sang <wsa@kernel.org>