i2c: brcmstb: allocate correct amount of memory for regmap

We want the size of the struct, not of a pointer to it. To be future
proof, just dereference the pointer to get the desired type.

Fixes: dd1aa252 ("i2c: brcmstb: Add Broadcom settop SoC i2c controller driver")
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org