staging: comedi: serial2002: fix different address space sparse warnings

staging: comedi: serial2002: fix different address space sparse warnings

The struct file_operations (*read) and (*write) operations expect the
buffer to be a __user space pointer.

Currently the (*write) operations in this driver cause this warning:
warning: incorrect type in argument 2 (different address spaces)
  expected char const [noderef] <asn:1>*<noident>
  got unsigned char [usertype] *buf

And the (*read) operations cause this warning:
warning: incorrect type in argument 2 (different address spaces)
  expected char [noderef] <asn:1>*<noident>
  got unsigned char *<noident>

Use __force to cast the buffer to a __user pointer to suppress the
warnings.

Consolidate the (*read) calls into a helper function, __tty_readb().

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>