mxser: simplify mxser_ioctl_op_mode

* ModeMask local array is just inverted OP_MODE_MASK shifted by shiftbit.
  Drop this array and use the value directly.
* return from the if's true branch and drop 'else' branch by moving the
  code completely outside the if.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-40-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>