regmap: spmi: Fix regmap_spmi_ext_read in multi-byte case

Specifically for the case of reads that use the Extended Register
Read Long command, a multi-byte read operation is broken up into
8-byte chunks.  However the call to spmi_ext_register_readl() is
incorrectly passing 'val_size', which if greater than 8 will
always fail.  The argument should instead be 'len'.

Fixes: c9afbb05 ("regmap: spmi: support base and extended register spaces")
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org