i2c: pxa: consolidate i2c_pxa_*xfer() implementations

Most of i2c_pxa_pio_xfer() and i2c_pxa_xfer() are identical; the only
differences are that i2c_pxa_pio_xfer() may reset the bus, and they
use different underlying transfer functions. The retry loop is the
same. Consolidate these two functions.

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