iicbb: allow longer SCL low timeout and other improvements

First, SCL low timeout is set to 25 milliseconds by default as opposed
to 1 millisecond before. The new value is based on the SMBus
specification. The timeout can be changed on a per bus basis using
dev.iicbb.N.scl_low_timeout sysctl.

The driver uses DELAY to wait for high SCL up to 1 millisecond, then it
switches to pause_sbt(SBT_1MS) for the rest of the timeout.

While here I made a number of other changes. 'udelay' that's used for
timing clock and data signals is now calculated based on the requested
bus frequency (dev.iicbus.N.frequency) instead of being hardcoded to 10
microseconds. The calculations are done in such a fashion that the
default bus frequency of 100000 is converted to udelay of 10 us. This
is for backward compatibility. The actual frequency will be less than a
quarter (I think) of the requested frequency.

Also, I added detection of stuck low SCL in a few places. Previously,
the code would just carry on after the SCL low timeout and that might
potentially lead to misinterpreted bits.

Finally, I fixed several style issues near the code that I changed.
Many more are still remaining.

Tested by accessing HTU21 temperature and humidity sensor in this setup:

superio0: <Nuvoton NCT5104D/NCT6102D/NCT6106D (rev. B+)> at port 0x2e-0x2f on isa0
gpio1: <Nuvoton GPIO controller> at GPIO ldn 0x07 on superio0
pcib0: allocated type 4 (0x220-0x226) for rid 0 of gpio1
gpiobus1: <GPIO bus> on gpio1
gpioiic0: <GPIO I2C bit-banging driver> at pins 14-15 on gpiobus1
gpioiic0: SCL pin: 14, SDA pin: 15
iicbb0: <I2C bit-banging driver> on gpioiic0
iicbus0: <Philips I2C bus> on iicbb0 master-only
iic0: <I2C generic I/O> on iicbus0

Discussed with: ian, imp
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D22109