mtd: spi-nor: Fix Cadence QSPI page fault kernel panic

The current Cadence QSPI driver caused a kernel panic sporadically
when writing to QSPI. The problem was caused by writing more bytes
than needed because the QSPI operated on 4 bytes at a time.
<snip>
[   11.202044] Unable to handle kernel paging request at virtual address bffd3000
[   11.209254] pgd = e463054d
[   11.211948] [bffd3000] *pgd=2fffb811, *pte=00000000, *ppte=00000000
[   11.218202] Internal error: Oops: 7 [#1] SMP ARM
[   11.222797] Modules linked in:
[   11.225844] CPU: 1 PID: 1317 Comm: systemd-hwdb Not tainted 4.17.7-d0c45cd44a8f
[   11.235796] Hardware name: Altera SOCFPGA Arria10
[   11.240487] PC is at __raw_writesl+0x70/0xd4
[   11.244741] LR is at cqspi_write+0x1a0/0x2cc
</snip>
On a page boundary limit the number of bytes copied from the tx buffer
to remain within the page.

This patch uses a temporary buffer to hold the 4 bytes to write and then
copies only the bytes required from the tx buffer.

Reported-by: Adrian Amborzewicz <adrian.ambrozewicz@intel.com>
Signed-off-by: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>