usb: uhci: Work around bogus clang shift overflow warning from DMA_BIT_MASK(64)

usb: uhci: Work around bogus clang shift overflow warning from DMA_BIT_MASK(64)

After commit 18a9ec886d32 ("usb: uhci: Add Aspeed AST2700 support"),
clang incorrectly warns:

  In file included from drivers/usb/host/uhci-hcd.c:855:
  drivers/usb/host/uhci-platform.c:69:32: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
     69 | static const u64 dma_mask_64 = DMA_BIT_MASK(64);
        |                                ^~~~~~~~~~~~~~~~
  include/linux/dma-mapping.h:93:54: note: expanded from macro 'DMA_BIT_MASK'
     93 | #define DMA_BIT_MASK(n) (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
        |                                                      ^ ~~~

clang has a long outstanding and complicated problem [1] with generating
a proper control flow graph at global scope, resulting in it being
unable to understand that this shift can never happen due to the
'n == 64' check.

Restructure the code to only use DMA_BIT_MASK() within
uhci_hcd_platform_probe() (i.e., function scope) to avoid this global
scope issue, similar to the approach of commit 274f2232a94f ("usb: ehci:
Add Aspeed AST2700 support").

Closes: https://github.com/ClangBuiltLinux/linux/issues/2136
Link: https://github.com/ClangBuiltLinux/linux/issues/92
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Ryan Chen <ryan_chen@aspeedtech.com>
Link: https://patch.msgid.link/20251015-usb-uhci-avoid-bogus-clang-shift-warning-v2-1-68532d2f6114@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>