regmap-i2c: Subtract reg size from max_write

Currently, when an adapter defines a max_write_len quirk,
the data will be chunked into data sizes equal to the
max_write_len quirk value.  But the payload will be increased by
the size of the register address before transmission.  The
resulting value always ends up larger than the limit set
by the quirk.

Avoid this error by setting regmap's max_write to the quirk's
max_write_len minus the number of bytes for the register and
padding.  This allows the chunking to work correctly for this
limited case without impacting other use-cases.

Signed-off-by: Jim Wylder <jwylder@google.com>
Link: https://msgid.link/r/20240523211437.2839942-1-jwylder@google.com
Signed-off-by: Mark Brown <broonie@kernel.org>