PCI: mediatek: Fix memory mapped IO range size computation

Mediatek's HW assigns a MMIO address range (typically starts from
0x20000000 to 0x2fffffff for both mt2712 and mt7622) for PCI usage.

This MMIO address space represents the address space that can
be allocated to PCI devices through Base Address Registers.

Even though the full MMIO address range is available to be allocated, it
should be enabled by the PCIE_AHB_TRANS_BASE register in the host
controller and the size that is enabled is determined by AHB2PCIE_SIZE
bits in this register.

Owing to a bug in the MMIO window size computation, current code does
not enable the full size of the available MMIO address range in the
PCI host controller; if the PCI devices BARs requested size exceeds the
size enabled through the PCIE_AHB_TRANS_BASE register the requests
targeting the disabled address address space will be blocked by the root
complex causing a system error.

Existing code has never run into a system error in production because
even half of the enabled MMIO range (128MB) is big enough for typical
devices BAR requests (4MB) but the full MMIO address range should
be enabled regardless.

Fix the MMIO window size computation by using resource_size(mem) instead
of mem->end - mem->start.

Since the MMIO window size for both MT2712 and MT7622 is 0x10000000,
this change will update the parameter passed to fls() from 0xfffffff to
0x10000000 and calculate the whole memory mapped IO range size
correctly.

Detected through coccinelle semantic patch (and related warning):

scripts/coccinelle/api/resource_size.cocci:

pcie-mediatek.c:720:13-16: WARNING: Suspicious code. resource_size is maybe missing with mem

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[lorenzo.pieralisi@arm.com: rewrote the commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>