mtd: st_spi_fsm: Refactor status register operations

This patch refactors the fsm_read_status() and fsm_write_status() code to
support 1 or 2 byte operations, with a specified command.  This allows us to
remove device/register specific code, such as the N25Q fsm_wrvcr() function.

The 'QE' configuration code is updated accordingly, with minor tweaks to ensure
the register values are only written if actually required.  One notable change
in this area is that the 'W25Q_STATUS_QE' bit-field is now defined with respect
to the 'SR2' register, rather than the combined 'SR1+SR2' register which is only
used for write operations.

Signed-off-by: Angus Clark <angus.clark@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>