i2c: mediatek: Fix i2c_spec_values description

The struct i2c_spec_values have it's members documented but is
missing the starting '@', which leads to warings like:

drivers/i2c/busses/i2c-mt65xx.c:267: warning: Function parameter or member 'min_low_ns' not described in 'i2c_spec_values'

We also delete min_high_ns member as it is not used in the code.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Qii Wang <qii.wang@mediatek.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>