i2c: pxa: re-arrange functions to flow better

Re-arrange the PXA I2C code to avoid forward declarations, and keep
similar functionality (e.g. the non-IRQ mode support) together. This
improves code readability.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Wolfram Sang <wsa@kernel.org>