i2c: spacemit: move i2c_xfer_msg()

i2c: spacemit: move i2c_xfer_msg()

The upcoming PIO support requires a wait_pio_xfer() helper, which is
invoked from xfer_msg().

Since wait_pio_xfer() depends on err_check(), move the definition of
xfer_msg() after err_check() to avoid a forward declaration of
err_check().

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Alex Elder <elder@riscstar.com>
Signed-off-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20260207-b4-k3-i2c-pio-v7-1-626942d94d91@linux.spacemit.com