i2c: bcm2835: Add debug support

This adds a debug module parameter to aid in debugging transfer issues
by printing info to the kernel log. When enabled, status values are
collected in the interrupt routine and msg info in
bcm2835_i2c_start_transfer(). This is done in a way that tries to avoid
affecting timing. Having printk in the isr can mask issues.

debug values (additive):
1: Print info on error
2: Print info on all transfers
3: Print messages before transfer is started

The value can be changed at runtime:
/sys/module/i2c_bcm2835/parameters/debug

Example output, debug=3:
[  747.114448] bcm2835_i2c_xfer: msg(1/2) write addr=0x54, len=2 flags= [i2c1]
[  747.114463] bcm2835_i2c_xfer: msg(2/2) read addr=0x54, len=32 flags= [i2c1]
[  747.117809] start_transfer: msg(1/2) write addr=0x54, len=2 flags= [i2c1]
[  747.117825] isr: remain=2, status=0x30000055 : TA TXW TXD TXE  [i2c1]
[  747.117839] start_transfer: msg(2/2) read addr=0x54, len=32 flags= [i2c1]
[  747.117849] isr: remain=32, status=0xd0000039 : TA RXR TXD RXD  [i2c1]
[  747.117861] isr: remain=20, status=0xd0000039 : TA RXR TXD RXD  [i2c1]
[  747.117870] isr: remain=8, status=0x32 : DONE TXD RXD  [i2c1]

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>