fbdev/ps3fb: fix compile warning

Commit 11bd5933 ("fbdev/ps3fb: use vm_iomap_memory()") introduced
the following warning:

drivers/video/ps3fb.c: In function 'ps3fb_mmap':
drivers/video/ps3fb.c:712:2: warning: suggest parentheses around '+' inside '<<' [-Wparentheses]

Fix this by adding the parentheses.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>