Implement changes based on Kees comments on the code review:
http://gerrit-minix.few.vu.nl/#/c/676/
Changes:
- use spin API instead of micro_delay() for busy-wait loops.
- use read16/write16/set16 from mmio.h to access I2C registers.
- reduce the timeout for soft reset.
- in read/write, don't mix variable declaration and initialization.
- after transfer, wait for access ready instead of blindly delaying.
- rename constants am335x, dm37xx to AM335X_I2C_BUS, DM37XX_I2C_BUS.
- rename ADDRESS_MASK to MAX_I2C_SA_MASK for clairity.
- rename omap_i2c_claim_bus() to omap_i2c_bus_is_free().