driver: wifi: eswifi: Fix spi buffer length

The eswifi spi driver sets up spi buffer length as number of frames,
but the length shall be number of bytes. Because eswifi use 16 bit as
frame size, so this turns out reading and writing half of data and
fails to sending any at command request and getting any responds from
eswifi module.

Fix it by setting up length as number of bytes.

Fixes #62056

Signed-off-by: Chien Hung Lin <chienhung.lin.tw@gmail.com>