evdev: fix array size calculation in decode_bitset_

evdev: fix array size calculation in decode_bitset_

max_nr is in bits (as it is a number of flags), result is in bytes, and
the array allocation has to be in personality words.

There's still an open question, however, what to do on big-endian
architectures when a non-divisible-by-4 value is returned.

* evdev.c (decode_bitset_): Declare size_bits, initialise it and use it
later instead of size; round up size by personality's word boundary.