staging: sm750fb: Remove set but not used variable 'actual_mx_clk'

staging: sm750fb: Remove set but not used variable 'actual_mx_clk'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/staging/sm750fb/ddk750_chip.c: In function set_chip_clock:
drivers/staging/sm750fb/ddk750_chip.c:59:15: warning: variable actual_mx_clk set but not used [-Wunused-but-set-variable]

It is not used since commit f0977109a577 ("staging:
sm750fb: lower case to fix camelcase checkpatch warning")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/1570520515-2186-4-git-send-email-zhengbin13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>