i2c: pxa: use official address byte helper

i2c-pxa was created before i2c_8bit_addr_from_msg() was implemented,
and used its own i2c_pxa_addr_byte() which is functionally the same.
Sadly, it was never updated to use this new helper. Switch it over.

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