wil6210: validate wil_pmc_alloc parameters

num_descriptors and descriptor_size needs to be
checked for:
1) not being negative values
2) no overflow occurs when these are multiplied
together as done in wil_pmc_read.
An overflow of two signed integers is undefined
behavior.

Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>