commands/setpci: Honor write mask argument

commands/setpci: Honor write mask argument

In the case that one passes a write mask with ":" the write_mask is
obtained from grub_strtoul() and then promptly overwritten by 0xffffffff
three lines later.

This appears to have been so since the initial version of setpci in 2009.
I'm surprised no one else has hit this issue in the past 12 years...

Signed-off-by: Wouter van Kesteren <woutershep@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>