regmap: Just send the buffer directly for single register writes

When doing a single register write we use work_buf for both the register
and the value with the buffer formatted for sending directly to the device
so we can just do a write() directly. This saves allocating a temporary
buffer if we can't do gather writes and is likely to be faster than doing
a gather write.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>