main/postmarketos-mkinitfs: fix device sed regex (MR 1436)

The p? is not redundant. Without it, the p in mmcblk0p2 for example does
not get cut off, meaning the resulting device is not "mmcblk0" but
"mmcblk0p". My bad, sorry for the breakage.

Fixes: 9f6600ba ("main/postmarketos-mkinitfs: rm pmOS_deleteme")
Fixes: 9d86f6fe ("main/postmarketos-mkinitfs: resize: unallocated space check, even if forced")