change type of force_io to bool

v2: Fix the type warning the other way round as proposed by Bjorn Mork

Fix for the following warning:
  CC [M]  /home/jmayer/work/thinkpad/git/thinkpad_ec.o
In file included from /usr/src/linux-3.14.1-1.geafcebd/include/linux/module.h:17:0,
                 from /home/jmayer/work/thinkpad/git/thinkpad_ec.c:33:
/home/jmayer/work/thinkpad/git/thinkpad_ec.c: In function ‘__check_force_io’:
/usr/src/linux-3.14.1-1.geafcebd/include/linux/moduleparam.h:349:45: warning: return from incompatible pointer type [enabled by default]
  static inline type *__check_##name(void) { return(p); }
                                             ^
/usr/src/linux-3.14.1-1.geafcebd/include/linux/moduleparam.h:395:35: note: in expansion of macro ‘__param_check’
 #define param_check_bool(name, p) __param_check(name, p, bool)
                                   ^
/usr/src/linux-3.14.1-1.geafcebd/include/linux/moduleparam.h:127:2: note: in expansion of macro ‘param_check_bool’
  param_check_##type(name, &(value));       \
  ^
/home/jmayer/work/thinkpad/git/thinkpad_ec.c:100:1: note: in expansion of macro ‘module_param_named’
 module_param_named(force_io, force_io, bool, 0600);
 ^

Signed-off-by: Joerg Mayer <jmayer@loplof.de>