spi: aspeed: Fix __iomem annotation and VLA parameter

Chin-Ting Kuo <chin-ting_kuo@aspeedtech.com> says:

This series fixes two sparse warnings reported by the kernel test robot.
The first patch fixes missing __iomem annotation on an MMIO pointer
parameter, which also caused a redundant cast at the call site.
A VLA function parameter warning is also fixed in this patch series.

Link: https://patch.msgid.link/20260522071621.102507-1-chin-ting_kuo@aspeedtech.com