omap: iovmm: disallow mapping NULL address when IOVMF_DA_ANON is set

commit c7f4ab26e3bcdaeb3e19ec658e3ad9092f1a6ceb allowed mapping the NULL
address if da_start==0, which would then not get unmapped. Disallow
this again if IOVMF_DA_ANON is set. And spell variable 'alignment'
correctly.

Signed-off-by: Michael Jones <michael.jones@matrix-vision.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>