amiflop: fold headers into C file

amifd.h and amifdreg.h are only used from amiflop.c, and they're pretty
small, so move the contents to amiflop.c and get rid of the .h files.
This is preparation for adding a struct blk_mq_tag_set to struct
amiga_floppy_struct.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>