PCI: mediatek: Fix operator precedence in PCIE_FTS_NUM_L0 macro

PCI: mediatek: Fix operator precedence in PCIE_FTS_NUM_L0 macro

The original PCIE_FTS_NUM_L0(x) macro was buggy due to improper operator
precedence, where ((x) & 0xff << 8) was evaluated as ((x) & 0xff00).

Instead of just fixing the parentheses, use the standard FIELD_PREP()
macro. This makes the code more robust by automatically handling masks
and shifts, while also adding compile-time type and range checking to
ensure the value fits within PCIE_FTS_NUM_MASK.

Fixes: 637cfacae96f ("PCI: mediatek: Add MediaTek PCIe host controller support")
Signed-off-by: Li RongQing <lirongqing@baidu.com>
[mani: added the bitfield header include spotted by Sashiko]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Link: https://patch.msgid.link/20260515005552.2343-1-lirongqing@baidu.com