Staging: hv: mousevsc: Inline the code for reportdesc_callback()

Inline the code for reportdesc_callback() as this function is called from
mousevsc_probe(). As part of this, cleanup the code in reportdesc_callback().

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>