ewah: use less generic macro name

ewah: use less generic macro name

The ewah/ewok.h header pollutes the global namespace with
"BITS_IN_WORD", without any specific notion that we are
talking about the bits in an eword_t. We can give this the
more specific name "BITS_IN_EWORD".

Signed-off-by: Jeff King <redacted>
Reviewed-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>