drivers: sensor: st: add missing buffer size config

Previously, the Kconfig option CONFIG_STMEMSC_I3C_I2C_WRITE_BUFFER_SIZE
has been introduced to replace i2c_burst_write with i2c_write using a
buffer combining the address and data.
See commit 5f8da77ad3eaf0593107fd99253c1c13b3a98de0

Add missing buffer size config to stmemsc_i2c_write_incr to replace
fixed buffer size.

Signed-off-by: Jonas Spinner <jonas.spinner@burkert.com>