staging: xgifb: rewrite the right hand side of an assignment

This patch rewrites the right hand side of an assignment for
expressions of the form:
a = (a <op> b);
to be:
a <op>= b;
where <op> = << | >>.

This issue was detected and resolved using the following
coccinelle script:

@@
identifier i;
expression e;
@@

-i = (i >> e);
+i >>= e;

@@
identifier i;
expression e;
@@

-i = (i << e);
+i <<= e;
Signed-off-by: Aya Mahfouz <mahfouz.saif.elyazal@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>