drivers: i2c: nrfx_twim_rtio: cast buf to non-const

RTIO correctly declares tx buffers as const, however, the
existing I2C API and NRFX (SDK) drivers don't. Therefor cast the
const buf to non-const when passing the tx buf to the twim driver
from the RTIO call to avoid the const warning.

The tx buffer is being treated as const data naturally in the
NRFX driver, its just not declared as such since we reuse the
buffer for both RX and TX data.

Alternatively the SDK and "shim" drivers built on top of it need
to be updated, which is quite a bit of work :)

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>