staging: rtl8712: style fix multiple line dereferences

This patch fixes various coding style issues in the rtl8712 module as
noted by checkpatch.pl related to dereferencing over multiple lines.

It fixes the following checkpatch.pl warning:

WARNING: Avoid multiple line dereference - prefer %s

Signed-off-by: Martin Homuth <martin@martinhomuth.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>