regmap: spi: Reserve space for register address/padding

In the Linux kernel, the following vulnerability has been resolved:

regmap: spi: Reserve space for register address/padding

Currently the max_raw_read and max_raw_write limits in regmap_spi struct do not take into account the additional size of the transmitted register address and padding. This may result in exceeding the maximum permitted SPI message size, which could cause undefined behaviour, e.g. data corruption.

Fix regmap_get_spi_bus() to properly adjust the above mentioned limits by reserving space for the register address/padding as set in the regmap configuration.