staging: sm750fb: fixed a assignment typo

fixed a typo issue in get_mxclk_freq() function.

the original code using PLL_CTRL_M_SHIFT for shifting to set N flag.
which is not right, it should be PLL_CTRL_N_SHIFT.

both PLL_CTRL_M_SHIFT and PLL_CTRL_N_SHIFT
  defined in drivers/staging/sm750fb/ddk750_reg.h

Signed-off-by: Lynn Lei <lynnl.wit@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>