i2c: aspeed: Add an explicit type casting for *get_clk_reg_val

This commit fixes this sparse warning:
drivers/i2c/busses/i2c-aspeed.c:875:38: warning: incorrect type in assignment (different modifiers)
drivers/i2c/busses/i2c-aspeed.c:875:38:    expected unsigned int ( *get_clk_reg_val )( ... )
drivers/i2c/busses/i2c-aspeed.c:875:38:    got void const *const data

Reported-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>