staging: comedi: comedi_fops: cleanup comedi_mmap()

Use comedi_dev_from_minor() to simplify the return -ENODEV tests.

Change the (foo == NULL) tests to simply (!foo).

Use a local variable to hold a pointer to the async->buf_page_list[]
when doing the remap_pfn_range(). This cleans up the ugly line breaks
for the page_to_pfn(virt_to_page(...) operation.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>